I found a huge pack of shells in the crafts department at Walmart. They were many different sizes, colors and shapes. Its great fun to watch them change shells!!! Just be sure that the shells are natural and don't have any varnish to them. It's also good to boil them for a few mintues before putting them in the tank (I foudn that one out after I dropped mine in) Good luck!
Marlin: Okay, a mollusk walks up to this sea cucumber, well he doesn't actually walk, he's just there, and he turns to the sea cucumber, and... Well, wait, there's a mollusk and a sea cucumber and...
Chum: You know for a clown fish, he's not that funny.