Costume shops sell fancy-dress clothing for wear during Fasching or Halloween.
- Halloween Gore Store - a Halloween costume store in Sendlinger Tor area.
- Hera Rauch - costume shop
- Kostüm Rainer Stegemann - high end theatre quality
All the big department stores have sections devoted to Fasching costumes, but this is seasonal:
And Aldi have been known to sell very cheap wigs. But there's no predicting which particular branches and when they will have stock in.
C&A and Woolworth's are both really cheap too.
If you've got an imagination, Kleidermarkt im Tal have lots of second-hand clothing that can easily be made into cheap costumes.
See also the related TT chat topic: Halloween costume shops