I would guess any place that sells snails. I have a 3lb bag coming from florida I ordered off ebay. (more exotic shells)

http://stores.ebay.ca/Wagners-Southe...QQftidZ2QQtZkm

Little bit old thread here guys.. LOL.. but anywho, if they have enough empty shells available to them, they dont cause that much a problem for snails, as my snail is fin with my hermits.

I bought my empty shells at the local Total Pet. But just any kind of normal sea shells works, just make sure they arent painted, cause that can release toxins thatll kill stuff.

Go to your local walmart and check in the craft section.
They usually have assorted bags for cheap.