Go to cichlid breeding.com a great site and lots of shells to choose from. Some people prefer dollar store shells or "craft" store shells, me I buy them online, I like the selection.
I've heard of people buying the shells that are used to serve escargot from amazon as well.
I tend to buy empty turbo snail shells from my LFS. I boil them to make sure that I get any dead creatures out of them and then put them in my tank when the shells have cooled down.
Arlene