There are various websites that sell shells, they aren't specifically for aquaria but they'll work just fine. Your best bet would be to buy various sized cerith shells. Some species like the knobby cerith would fit the larger cortez hermit crabs, I presume. In addition, other species of cerith snails are tiny enough for even the smallest hermit crabs. If you live in Florida, it'd probably be easy to find a sea shell store. Otherwise, you're going to have to buy from an online sea shell store. In large amounts, it is pretty cheap per shell. If you have other types of hermit crabs, like the halloween hermit crabs, you may want to buy those ones too.