Where can I find acetate sheets for scrapbooking?

I want to try using acetate sheets for making invitations and for general scrapbooking thigs but I cant seem to find those acetate sheets. Is there something else that can be used that will have the same effect as the acetate?

Around here we have specility craft stores that carry just about any scrapbooking supplies. You can also try scrapbook.com .. they have a store there and you can ofder them. Or try e-bay. sometimes you can find them there too.