I get mine at the ocean, but if you're landlocked, try craft stores like Michael's or jewelery/rock shops that that carry fossils, polished stones, etc.
Personally, I feel that once they get too big for their shells, then they're too big for my tank. Because regardless of empty shells in the tank, they will still want that one that is already occupied by your snails. I removed my hermits a long time ago and banished them to the sump. Keeps the sump pretty clean for me.
