My favourate price is free, so I go to the large chain LFS and dip into their apple snail tanks and take empty shells. They don't mind, they'd throw them out anyway, and they work perfectly. Escargot shells work well too. It's best to try for a variety of sizes to let the shellies choose their favourates. Different shelldwellers need different amount of shells. On both extremes Brevis need only a couple, Multifasciatus like as many as you are willing to add. Most fall somewhere in between.
The real Neothauma shell, which they use in the wild