I got my sintra from a sign shop. Just look up signs in the yellow pagers and make some calls. I'm sure someone in your area has some small scraps they would sell. I got 4'X4' worth for less than $20.
Check out this link and see if they have a distribution/retail office near you. I found one in Fresno, California (1 hour away from where I live) walked into the sales office and they had no problem selling me a single sheet. I even told the guy it was for a costume and he became interested. On their website they refer to what we call sintra as Polystyrene(high impact) Generic Print Grade, so keep that in mind. I had to create an account in order to look up specs and prices online, but it doesn't cost anything and it's nothing more than public information anyways(address, full name, username, password, etc.). And I listed my business as costume making if they asked(can't remember).
You can try getting free sintra from college science labortories. Also the folks at fram shops have extra scraps of the stuff in large piles. Some of the stuff is better quality than what you would get at Home Depot.