This is from last year's information, reposted for those that are still looking for costumes.
Where to Buy Costumes:
1. There is a group of "holiday" stores along Chungshan West Road, just north of Taipei Station. They change their themes throughout the year, so now is a good time for Halloween and Christmas.
2. Lococo is a very large outlet store, with some fantastic costumes available all year round for good prices. We just got a beautiful princess dress for my daughter for TWD290. I also picked up some nice Old Navy sweater jackets in preparation for the winter. Oh, and they have real ballet outfits for just TWD100!
Beitou Road Sec. 1, No. 7 (Beitou District). This is just off the MRT line, so not as hard to get to as it sounds.
3. Best Buy up on Section 7 of Chungshan North Road. If you walk up the hill north of Taipei American School (north side of Tienmu Road), there are several outlet stores that carry costumes. You can get some really good deals very close to and just after Halloween.