Escargot shells: cheap and somewhat easy to obtain, good size too. I use these in my occie tank
Mexican turbo snail shells: you can get them from saltwater fish stores for free typically, just have to wash them very well, and find the larger ones. Also, dead turbo snail smells god awful, like vomit inducing. I use these in my multi tank
Whale's eye shell: expensive but look pretty cool. I have a couple in one tank to provide a larger shell.
Neothauma snail shell: the real deal. If you can find these, it's the one real way to do a true shellie biotope. They're expensive, and near impossible to find larger quantities
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