The best place/time to find venom crabs is when you first leave the monastery for Arnika. Their appearance is random, so you should save before leaving. If no crabs show up by the time you pass under the stone arch, restore and try again.
Of course, that's a little late for you, unless you want to start over again with your party. I very rarely see venom crabs once the monastery is behind me. Bayjin usually has curare crabs, which aren't the same thing (venom's big brother, but no shells). I can't recall offhand seeing any in the Sea Caves area, either.
Nightowl2
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