I just go to the local LFS superstore on a busy day and go for a dive in their apple snail tank, there are always a good number of empty ones in there - I've gotten about 30 or 40 of them this way, and they're free!
Authentic Neothauma shells can be ordered online, but they are not cheap, and they don't do anything that other snail shells don't. Depending what you are using them for you can also use escargot shells.