Almost nobody rents children's costumes--kids being the messy and unreliable creatures that they are -- however stores listed here not only rent adult costumes but often sell children's costumes at fairly low prices ($6.95 and up).
Addresses can be found under "Costumes" in the Yellow Pages: District
Al's Magic Shop. Also a good source for accessories such as masks and costume kits.
Costume Studio. Rentals only from this Capital Hill shop.
Deja Vu. Clothes from 1900-1950 a specialty.
Geraldine's. Costumes replace vintage clothing in October. (Indiana Jones among them.)
Overflow II. Costumes, vintage clothing, and the Jack Mullane stock (picked up when that store closed).
Ronna Costumers. Standard downtown source: convenient but a little on the high side. Maryland
Barry's Magic Shop, Wheaton. Rentals include a 6-foot rabbit; sales include a cardboard haunted house--for your front porch--that you enter through a coffin.
Costumes Creative, Bethesda. Why be Judy Garland when you can be a bunch of grapes or three peas in a pod?
Dance Center Theatrical, New Carrolton. Be a skunk or a bumblebee with kits and makeup from this theatrical supply shop.
Dancer, Camp Springs. Rentals are secondary to fabrics and trims for do-it-yourselfers.
Gene's Costumes, Wheaton. Choose a team from a wide selection: Merlin and Morgan le Fay, Abelard and Heloise, Smokey and the Ranger, Henry VIII and his wives, a lion and his tamer.
Plaza Costumes, College Park. Limited stock, but Wonder Woman is on the list.
Repeat Performance, Rockville. When else can you be a pink gorilla or a pumpkin? Virginia
Back Stage Dance & Gym Supplies, Occoquan. The elephant is already rented, but the lobster, Maryland crab, and big bad wolf might be available.
Fantasy Costumes, Alexandria. Kangaroos are new with this major costumer, whose Chinese emperor's coat from the last royal court of China (security deposit $200) has never been rented.
Kinsley's Theatrical Costumes & Supplies, Rosslyn. Egyptian, Greek, Turkish (etc.) folk costumes and gypsy outfits.
Masters Costumes, Bailey's Crossroads. Theatrical costumes that can turn you into King Tut, Napoleon and Josephine, or a lion.
National Capital Historical Sales, Springfield. Old U.S. and foreign military costumes.
Costumes Creative, Costume Studio, Dancer, Fantasy Costumes, Gene's, Masters and Overflow II do custom-designing. (Too late for this year, but how about getting an earlier start next year?)