Usually your LFS will have some, just ask. I keep on the average 1.5 - 2 extra shells for every hermit. Varying sizes and types are important.
At a local party supply store, I got a small basket of assorted shells for $2! Go to the hawiian luau section. Ive actually got a scarlet reef hermit living in a beautiful cowrie shell that came in this basket. Check it out!