Not every one has the time to make a halloween costume, so if you just want to go and buy or rent one, try one of these places:
1) Spotlight at Plaza Singapura
2) Toys R Us at Vivo City
3) Cold Storage at Siglap or Parkway Parade
4) Carrefour at Suntec City
5) Party City at Holland Village and Raffles City
6) Number one Costumes (Costumes costumes) at 32 Aliwal Street
7) House of Costumez at 1 Sophia Road, #-2-03 Peace Centre
8.) The Magic Wand at Tanglin Mall, Lucky Plaza, Suntec City and Funan Mall.
9) Corner shop in Holland Village (right by The Body Shop, near the top of the slope).
10) Foreigners House of Costumes at No. 49 Surin Ave.
Alternatively you can go to Arab street and buy the raw materials to make your own Halloween costume. Either way Happy Trick or Treating!