hmm stone is probably what I would suggest. Do they have anything on them like laquer or paint? Maybe you could try some plastic but I'd think they would be painted. hmm. maybe you could find a pottery buddha. Ask if it's vitrified. This means that it doesn't absorb water and as long as it has no glaze it should be ok. If i'm wrong about this people please correct me as I am no expert.
Asian theme, hmm..sounds interesting. I'd recommend stone too. Plastic may give algae problems. You don't wan't a green/brown Buddha, do you?
But then it will also depend on the type of plastic.