What happens if you tell the salesperson up front that the article of clothing is for a costume? At least it would explain why the white dress of death is a good thing. 
Granted, I've gotten some odd looks at Home Depot when I've explained that the PVC pipe will holding a werewolf instead of water.
I had a somewhat similar experience with PVC pipe. I was making items called "whisper phones" (they look sort of like old-fashioned telephone receivers). These allow students to read aloud to themselves in class by whispering into the phones, which magnify their voices in their own ears so they don't disturb other students.
(For teachers who might want to use this idea -- you put small jointed PVC pieces on the ends of 6" lengths, to make the mouth and ear parts.)
So I bought a 12-foot length of PVC and asked for it to be cut into 6" lengths. (I was happy to pay for the extra cuts, because I knew they gave you only so many for free.) You can imagine the puzzled and somewhat annoyed look I got from the salesperson -- until I explained what I was making.
