Tired of having your gowns and tuxedos custom made from the same old silk and satin? Weary of going to White House soirees in formal duds that look no different from those of the millionaire on your right? Salvation is as close as your nearest Salvation Army. Washington's second-hand clothing stores are not just for bowling shirts and blue suede shoes. There are fire-engine red, double-breasted dinner jackets waiting to be resurrected. Gold lame' evening gowns begging for one more twirl at another New Year's ball. With a bit of persistence and an eye for buried treasure, you can pick up an outlandish outfit for a modest price. "There are little holes all over it, but for $50 I can learn to live with it," said Susan Rasky, eyeing a muskrat coat with a lot of life left in it at Classic Clothing in Georgetown. Rasky, a Washington journalist on a recent Sunday safari for an evening bag, was bagged herself by the muskrat. "I've been dying for one of these," said Rasky. "But I wouldn't dream of buying a new one, which would mean killing an animal." There are more than two dozen second- hand stores in the Washington area that stock, at least occasionally, clothes that are still ready to boogie even if their original owners no longer are. Stores like the Blue Moon, Second Time Around, Value Village and American Rescue Workers cover a spectrum from high-priced chic to low-down funk. The other day for $35, you could pick up a white, three-piece tuxedo bordered in black and embroidered on an inside jacket pocket with the inscription "especially made for William Harris" at the American Rescue Workers at 716 Ritchie Road in Forestville, Maryland. At Amvets, a store at 6101 Georgia Avenue, a perfectly eye-blinding blue velvet tie was snatched up for a mere 95 cents. A floor-length gown with purple puckered sleeves, a black-and-yellow silk skirt with a sash to match, was a steal at $3.95. Feather boas, rhinestone tiaras and sequined tuxedo jackets appear then disappear from second-hand stores like so much cotton candy. "We don't have any trouble selling them," said Katherine Martin, manager of the American Rescue Workers thrift shop. "They go like hot cakes." Owners and managers of the second-hand emporiums say both the numbers and the amount of business have boomed recently, particularly since President Reagan and Nancy spruced up the social scene in Washington. "We sell an awful lot of stuff to people for inaugurations, Christmas and New Year's parties and debutante balls," said Canary Driller, sales clerk and stocker at Classic Clothing, which has three stores in the area. The best buys are often the result of luck. Two years ago, for instance, John Kehoe, a 31-year-old thrift-store devotee, was passing through Poolesville, Maryland, when he noticed a local thrift store advertising a sale. Kehoe walked away with a black tuxedo, lined in satin and obviously custom tailored. The price, 21/2 cents, was right. "I get the impression that the person who bought this wore it to some spectacular functions," said Kehoe, who sometimes wonders if his new duds are a bit disappointed in their new owner. "Not that I've disgraced it, but I think it had a once glorious past."