MTS/malaysian trumpet snails reproduce asexually and are live bearers but I understand that they may not be the best to feed to puffers because their shells are extremely thick and hard and can harm the puffers mouths. I've also heard of people feeding them without issues but I wouldn't risk it.
I think your best bet would be to get some common pond snails and set up a breeding colony. Ask at your LFS and they will usually give the things away and also check on aqua bid. I often see large lots of them listed for cheap.
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