You can get c. tigris and sometimes c. arabica at pet stores from time to time, but that's about it. Both of these aren't really suitable, they get big and bulldoze things all over the tank, and would probably starve to death in short order.
I have asked several serious shell collecters, and they pretty much agree that if you want live ones you'll have to collect them yourself.
If you find another source for live cowries please tell me i'm also interested.