THEY COME IN search of costumes - Halloween, a party, you know how it is - but they seek something much more. As October 31 draws near, people flock to the costumer's, hoping to find, somewhere in the gaudy satins, velvets and brocades, another self buried deep beneath convention and just beyond the grasp of their own imaginations. So they come to be transformed by the costummer's craft, for one glorious night, into an alter ego.

Washington's costume emporia range from suburban cottage industries to chic little in-town shops to expect-the-unexpected warehouses. No two shops are alike, which makes rummaging around for a rental all the more fun. For $15 you can wear the hoop skirt Anna Roosevelt wore to a White House party; for $150 you can be Darth Vader, complete with mylar push-button chest panel.

"It's like living in a fairy tale," says Couret Hubbard, who owns Mullane's. "Now those are fun to do. Only nowadays, most people haven't heard of fairytales." Most people haven't heard of Mullane's, either, but perhaps they should. Founded in 1927, in what was once the fashionable downtown shopping district, Mullane's is one of Washington's oldest extant costumeries - but not for long. For the line ends with Jack Mullane's daughter, Mrs. Hubbard, who, at 75, according to another local costumer, "has forgotten more than I'll ever know."

Stepping into Mullane's is like opening a very old music box. The faint gay tinkling evokes images of a more elegant era one can long for without having lived it. In Mullane's, paint is peeling from the high stamped-tin ceilings and Mrs Hubbard, a frail Lillian Gish of a woman, sits at her desk dwarfed by racks burgeoning with musty costumes. Whatever wall space isn't taken by shelves piled high with hats, helmets and papier-mache heads, is hung with faded brown photos of costumed revellers, whose everyday clothes would still be costumes to us. One particularly elaborate group shot is inscribed "John Hays Hammond house, Twelfth Night, 1927."

Back then, the Willard and all the big hotels had grand Halloween balls, not to mention the society parties, Mrs. Hubbard recalls wistfully. And they all came to Jack Mullane's.

"When the [Franklin] Roosevelts first moved into the White House, their daughter had a party. We fixed her costume - a great big hoop skirt. It's here someplace. I can still remember her children romping through the rooms when she tried it on."

Another framed photograph reveals young Fred and Adelle Astaire. The Mullanes were primarily "show people," Mrs. Hubbard explains with pride. Hailing from New Orleans, a city enchanted with Mardi Gras masquerades, they roamed the country with a traveling show in the early 1800s. Then came vaudeville. "My grandmother knew Little Sheba, now that's going back a ways yonder," Mrs. Hubbard remarks. But the Mullanes eventually wanted to settle down, send their daughter to school and lead normal lives, so they came to Washington and opened the shop.

Mrs. Hubbard eventually inherited the business and all the original costumes. These alone tell the story. There's not a zipper or scrap of polyester in here, just lots of hooks, stays and old lace. Mrs. Hubbard points out a Gay '90s aqua antique satin gown trimmed with embroidery, an 1880s bustled peach taffeta, a pink hoop skirt with a matching jacket trimmed with silver-blue roses and lace sleeves, and an 1890s velvet-edged dress with a sewn-in China silk corset - not one made on a push-button Singer. These are the real McCoys. Then there are the theatrical costumes like the 40-year-old Robin Hood suit, all lacing and green wool with sleeves fit for Errol Flynn to duel in, the silver-and-black paisley Elizabethan gown or the little velvet French Renaissance page jackets. But this is just the beginning. There are four stories filled with cartons and racks of jewel-encrusted satins and embroidered velvet - all for rent at $15.75 a month. Sadly, many are now frayed and as fragile as Mrs. Hubbard herself, gossamer echoes of faded laughter, clinking glasses and faraway music. "I'm afraid we'll just be a memory soon. I hope we're a good one," Mrs. Hubbard says sadly.

Washington's costume carriage trade that once went to Mullane's is now clearly the domain of Ronna's Active Costume Service. Located at the base of a glass-and-girder office building at 18th and M, Ruth and Nathan Sim's shop boasts the highest prices in town and the most famous clientele - they claim to have handled the White House since Kennedy.

A top-of-the line Darth Vader with mylar and push-button chest panel, a foxy black witch and red satin devil (all $150) greet you in the window, and inside is the wherewithal to turn anyone into a vampire, Indian, cowgirl, wizard, Eastern prince or the Jolly Green Giant, just to name a few.

"We do all our own designing, select the fabrics and have the costumes made in New York," Sims says as he points out a brilliant feathered and beaded voodoo costume that virtually shrieks Carnaval! "She stays up all night dreaming up things," he says, nodding toward his wife.

A great deal of individual attention is Ronna's key to success, Mrs. Sims says. "I look at a person and I know what they can wear. We always find the right costume for the right person. And we go out of our way not to send people dressed alike to the same party," she adds. How do they know who's going to what party?

"Experience. We just know," Mrs. Sims says with a sly wink.

For some renting a costume becomes an important part of the Halloween ritual, taking the edge off their embarrassment and lending a legitimacy to the whole thing. After all, if you're spending a lot of money on it, it can't be ridiculous, now can it? Plus there's the sheer delight of going on an ego trip around the world and through history in 80 chabges of costume, sometimes more. Costumer Jean Showalter of Gene's Costumes says she's had to throw people out of her shop at 11:30 on Halloween night, begging them to please go to their parties and let her go to bed.

"Face it, Halloween is fantasy time," says Showalter, who has about 2,000 costumes crammed into her suburban Wheaton basement. "I've had people in here for four or five hours trying on everything in the shop. They try on every fantasy they've ever had. They'll go from a princess to a call girl to a flapper to a genie to Judy Garland to Camelot," she says.

"People call up almost apologetically, saying they've never done this sort of thing before. Then they come in here and go wild."

Jean pulls a silk-backed black velvet gown with a peach-rayon-and-black-lace bodice off the rack. "This one goes outevery Halloween." She points to a cloud of aqua chiffon covered with a web of bugle beads and emeralds. "They put this on and they think they're Ginger Rogers," she says with a laugh.

Certain truths about human nature seem to emerge in the costumer's shop, Showalter's learned.

"We've found that with men, you show them a hat and nothing else matters. They see a hat and nothing else matters. They see those gladiator hats and the fact that they're going to be bare-legged with just a little skirt on doesn't seem to bother them," Jean says. Young women tend to throw caution and feminism to the winds as a glamorous harem girl or lady of the night in hopes of enticing Prince Charming, while women well into middle age strive to recapture the innocence of youth with a Raggedy Ann or impish elf costume.

Couples, too, follow a costume pattern traced by time. Young couples or newlyweds going to their first costume parties together choose to be romantic duos like Romeo and Juliet or Anthony and Cleopartra, but older couples usually go as a pair of clowns or Jack and Jill. Yet there are instances when what God has joined, a trip to the costumer's may read asunder. Jean recalls one couple in their 40s who drove all the way from the Eastern Shore to pick up Anne Boleyn and henry VIII costumes. He saw the Cowardly LIon costume and it was all over.

"He did Bert Lahr in here for two solide hours. He wouldn't the darn thing off. She was absolutely livid with rage. But they left here as Anne Boleyn and the Cowardly Lion."

Like most costumers, Jean's mainstay is the theater. Her husband, Chuck, directs community theater and Jean does the costuming. But come Halloween everything goes, and you can pick from Moliere's Miser to Auntie Mame. She also has togas. "Togas were actually 18 yards long and 60 inches wide. You just folded them in half and started draping. I made them that way for 'Julius Caesar' and the actors were ready to kill me, so I redesigned them to be more manageable.

Mrs. Showalter says that most of her crustomers are young people, so she trys to keep prices low - $15 to $45. "My most expensive thing is a suit of armor. That's $100 because it's a stage piece that I really don't want to be worn. I've got too much time and material tied up in it. It's all extruded plastic and the molds cost a fortune to make." Jean's made all her own costumes except for a line of Brooks Van Horn creations she bought from someone living the business. Brooks Van Horn, a New York house, is to costumers what Stradivarius is to violinists. Started in 1852, Brooks Van Horn is the oldest costumer in the country and the most famous.

If you can't get to Brooks Van Horn's but would like to see more of the most sumptuous costume collections this side of Hollywood, make posthaste to the Fantasy Costume warehouse on the edge of Alexandria. Last year Joy Nelson bought a 110-year-old costume business from a Norfolk family who'd brought it over from England. Whatever Mrs. Nelson lacks in experience, she makes up for with enthusiasm. She's spent the past year sorting the tens of thousands of costumes she now owns and has read voraciously on the subject.

"I kept coming across these yellow-and-green suits," says Mrs. Nelson "Finally I found a little one with a hole for a tail and I realized they belonged to a circus act." Another discovery was a crumbling note in a tuxedo vest pocket proclaiming that this was M. Anderson's wedding vest when he married Marion Thompson, Jan. 4, 1871.

Clearly in it more for love than for money, Mrs. Nelson plans to turn part of the warehouse into a costume museum so she can display antique lovelies too delicate to rent. But there's no shortage of rental costumes; Fantasy's got those by the armful, including the biggest selection of children's rentals in town. Don't be put off by the small arsenal and Nazi memorabilia up front - that's Replica Inc., Fantasy's warehouse-mate. The costumes start just beyond the counters of antique masks and Halloween favors and keep going. First there's the Greek and Roman sector, then early American, Civil War, musical comedies, then your ethnic section - Indian, German, Mexican and harem girls. Children's and animal suits are across the back, then the long costumes: clowns, Draculas, Elizabethan and on and on extending along the other side.

Although costumers themselves seldom dress up, they all sincerely enjoyed dressing up others. But a costumer's lot is no bed of silk roses. The costumes constantly have to be cleaned and maintained, and there's always the chance that someone might damage one or not bring it back.

"Then you're stuck," says Joan Pekin of Bethesda's Costumes Creative. "Take my satin cape, for example. Last year someone got into a coffin and tore it. They say, 'Oh, you can patch it in ten minutes.' Yeah, well there's no such thing as a ten-minute job, and once it's mended a costume's not as good as it was brand-new.

"People can be very inconsiderate. They throw up in costumes. Gorillas are the worst. A gorilla suit makes people lose they inhibitions; they do crazy things. Last year a guy brought me back a gorilla suit in a paper bag. A good gorilla costs $150. So you figure you've learned something. So now I never let a gorilla go out without a full deposit. But you only have one stinker for a hundred nice guys. People who get dressed up are generally fun people," she says.

FULL FANCY RENTALS

COSTUME STUDIO - 35 8th St. NE. 544-5843. Monday through Saturday, 9 to 6. $15 to $35.

COSTUMES CREATIVE - 5907 Avon Dr., Bethesd 530-6037. Hours by appintment only. $10 to $100.

DANCE CENTER THEATRICAL - Carrollton Mall, New Carrollton. 459-0400. 10 to 9 Monday through Saturday, 1 to 5 Sunday. $7 to $18.

DEJA VU - 1675 Wisconsin Ave. NW. 965-1988. Monday through Saturday 11 to 6:30, Sunday noon to 5. $5 to $25.

FANTASY COSTUMES - 610 Franklin St., Alexandria. 549-2509. $35 to $75. Monday through Friday 10 to 5, Saturday 10 to 4.

Gene's Costumes - 3903 Joliet St., Wheaton. 94-4111. Hours vary. $15 to $40.

JACK MULLANE INC. - 718 11th St. NW. ME 8-2442. Monday through Friday 9 to 3, Saturday to noon. All costumes $15.75.

O'HARA ENTERPRISES INC. - 111 N. Washington St., Rockville. 424-9050. Monday through Thursday and Saturday 9 to 5:30, Friday 9 to 9. $15 to $25.

RONNA'S ACTIVE COSTUME SERVICE - 1139 18th St. NW. 296-3430. Monday through Saturday 10 to 5:30. $35 to $150.

MASKS, MONSTERS & SPARE PARTS

AL'S MAGIC SHOP - 1205 Pennsylvania Ave. NW. 638-4241. Monday through Friday, 9 to 5:30, Saturday to 5. Geared to the lunatic fringe, Al's carries grease paint, pancake, spirit gum and all manner of detachable facial hair (all from $1 to $7), along with the card tricks, whoopee cushions and other gags. Al's personal favorite among his hundred or so grotesque masks is a French-made zippered Frankenstein ($42.50), but he predicts that this year's hot seller will be a rubber replica of Saturday Night Live's onehead ($17.50).

A&N TRADING CO. - 714 12th St. NW. 737-2545. Monday through Saturday 9 to 6; 8701 Flower Ave., Silver Spring. 589-2676. Monday throught Friday 9 to 9, Saturday to 6. Never underestimate the possibilities of Army-Navy surplus stores for Halloween. Here a 1920s coal-mining helmet ($8.95) can turn you into a Harlan County hero. Gas masks ($7.95), machetes ($3.99) and belts ($2.25) accessorize any outer-space creature. Or put together an like outfit from the military ribbons (40 cents) and fatigues (about $10).

ARTISTIC DANCE FASHIONS - 4915 Cordell Ave., Bethesda. 652-2323. Monday to Friday 10 to 5:30, Thursday to 8, Saturday to 5. Have a devil of a time creating your own kook from cartons of horns, tails, pitchforks and fangs (all about $1). Top of it off with crepe hair ($1.50), scars and wrinkles made from rigid collodion at the makeup counter or choose from about 40 masks (including a decent $7.90 Darth Vader).

THE BODY SCENTER - 1618 Wisconsin Ave. NW. 965-3404. Monday through Saturday 10 to 6. Glistening white porcelain Pierrot masks ($22) sport hand-painted toothy grins, black stars, rosy cheeks and trails of gaily colored ribbon. You may not be able to keep these masks on all night, but you sure can make an entrance. Although the appeal here is more to the olfactory, if you're going as an oatmeal cookie there's scent to add the perfect touch.

THE BUTTON SHOP - 725 11th St. NW. 628-2851. Monday through Saturday 9 to 6. Notions and fabric shops are another marvelous source for Halloween regalia. This one has yards and yards of spangles, pearl ropes and rainbows of sequins. Cases hold lily of the valley sprays, red roses and other artificial flora ($2.50 to $3.50). There are rhinestone tiaras ($7.95), shimmering shawls and lace mantillas ($3 and $4), and giant $2 plumes in peacock shads for aspiring d'Artagnans.

DEJA VU - (see above) All of this antique clothing shop's treasures can be rented for Halloween at a third of their price. So to go with tails and a top hat rette holder (both rent for about $5) that would turn Gatsby green with envy. Plus beaded bags, furs, feathers, jewels and I Love Lucy hats.

FANTASY COSTUMES - (see above) While sorting through their warehouse, Fantasy unearthed bunches of 1940s masks. Made of pre-plastic cotton, they have a soft feel and strikingly painted features. For $3 there's Dumbo or a gold oriental cat lady. An Eskimo made in occupied Germany is $10, and for $2.50 there's an exquisite silver eye mask trimmed with black feathers. Orange and black beckon from a counter filled with '30s and '40s Halloween decorations and party favors, most under $1. Above hang children's costumes of decades past ($3), strangely innocent compared to today's pint-sized slick superheroes.

KINETIC ARTISTRY - 7216 Carroll Ave., Takoma Park. 270-6668. Monday through Friday 11 to 6, Saturday to 5. Theatrical lighting is the chief concern here, but Kinetic also carries an ample line of stage makeup and will happily show you how to use it. If you want to come in with a bang, here's the place to get flash powder ($6.25 per two-ounce bottle). Or you might rent a cobwebbing machine ($15) for the true haunted house feel. The machine is a small propeller that fits over a cordless drill and whips diluted rubber cement into Charlotte's finest.

PWANUTS'N'PARTIES - 11419 Georgia Ave., Wheaton. 942-6886. Tuesday through Saturday 11 to 9, Sunday to 7. You can buy a 10-pound bag of fresh roasted peanuts to go with your Jimmy Carter mask ($11.95) or come in like the kids do to ogle masks like Uncle Ooze ($2 to $60) while having your candy corn and mellocreme pumpkins weighed out. This incredible combination candy store and fun shop features the most amazing array of masks, phantasmagorical, ethnic and political. There's JFK, Jackie and Ali ($2.49), and vampires, corpses, hags and werewolves in varying stages of rubberized decrepitude. Extras like vampire nails, blood and plastic bellies, butts and breasts ($3.99) round out the selection.

RONNA's ACTIVE SERVICE - (see above) In addition to their costumes, Ronna's rents and sells all sorts of headgear including a Fat Lady Valkyrie helmet with blonde braids ($15.95), a sequin Statue of Liberty crown ($50) and myriad jewelled turbans ($25 and up). There's also animal faces ($19.50) and over-the-head rubber masks ($15.95 to $75).

STEIN'S THEATRICAL & DANCE CENTER - 1180 N. Highland Ave., Arlington. 522-2660. Monday, Wednesday, Thursday and Friday 10 to 9, Tuesday and Saturday to 6, Sunday noon to 5. There's a small line of costumes for sale - basic togas, tunics, capes, etc. - but Stein's true splendor is the trimmings: shampoo-out hairspray in colors like green and purple, blood capsules, black stump and hook hands, biblical wigs and beards, conquistador helmets, King Tut masks, magic wands, samurai swords, space guns, oozing eyes, werewolf feet, all kinds of prosthetic probosci and even jack o'lantern pasties. There's also a fine selection of rubber masks (including Sadat and Brezhnev for about $20 apiece), children's costumes and dime-store masks.

BARGAIN-RATE BOOGEY MONSTERS

CLASSIC CLOTHING - 3906 Benning Road NE. 396-2210 and 396-3334. Monday to THursday 9 to 6, Friday and Saturday to 7. Higher-priced vintaged clothes fill the second floor of this Anacostia warehouse (army surplus on the first) but the third floor is devoted to good old rag-picking. Manager Brian Streidell has stocked two bins with black dresses ($5 each) especially for the season of the witch.

VALUE VILLAGE - 4618 14th St. NW. 829-5728. Monday through Saturday 9 to 8:45. A recent visit to this Nieman-Marcus of thrift shops turned up a '50s green chiffon party dress for a dime, a black flamenco-type dress for $1.45 and a baggy suit for $1.80. At those prices, who can cry over a little spilled vampire blood? Check the thrift shop nearest you for counterparts.

THE CHAINS - Dart Drug, Drug Fair, Kresge's, Murphy's, Woolworth's and so forth - you know what's closest to you. Dance the night away as John Travolta (as Vinnie Barberino, not Tony Manero, but why quibble?), Fonzie, the Six-Million-Dollar Man or the Bionic Woman, all for less than $3. A whole network's worth of superheroes, cartoon characters and sitcom stars are available in flame-retarded, non-toxic, brightly colored nylon and plastic at your favorite chain drug store or five and dime. They're seized for kids, but if you're short you can probably squeeze into a large. Less than a dollar buys plastic fangs, non-toxic face crayons and masks like the last-resort 19-cent Zorro mask.