The relocated Ruby's Party is open till 9-9.30pm these days. The shop's packed with holiday stuff, plus a bunch of adult and children's costumes, latex, deocrations, make-up... the works. Add Ruby's Official Account on WeChat at ruby64016323 to see what outfits they have (and a 20% discount). They do costume rental — half price back if you return it within 24 hours in a decent state. They'll kuaidi it to you if you don't feel like going out past Hongqiao Airport.
Photo by Ruby's Party
Another option is the market in Yuyuan on Fuyou Lu, east of Henan Nan Lu, the big commodities buildings; chintzier, cheaper, and probably won't let you do rental, but it's downtown and you get that gritty in-person shopping experience. Pawing at goods with clammy fingers, communicating with a shopkeeper using your mouth hole... the real body horror experience.
Photo by Patrick Cranley (Historic Shanghai)
Looking for somewhere to wear your Halloween costume? Here are dozens and dozens and dozens of Halloween events happening in Shanghai.