Another alternative is checking out the local craft store. I found a nice assortment at Wal-mart's craft department for fairly cheap. Just make sure the shells are natural and its recommended to boil them in hot water before placing them in your tank.
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.