I collect snails from my back garden and feed to my blue tongues. I don't use any garden chemicals however.
You could try freezing them. Collect a big batch of them, keep them in an old tank for two or three days, so that they void their stomach contents, and then put them straight in the freezer in a plastic box. My skinks take these (once defrosted) really well, but they are not so keen on the de-shelled Can o Snails, which are really expensive anyway (at least in the UK).
This doesn't get round the issue of chemical contamination, but would kill any worms/parasites.
As an aside, I really don't think that any parasite in a wild garden snail from Europe or North America could infect tropical/semi-tropical reptiles from Australasia. But the freezing method is really good for storing snails, and if you have friends/neighbours who want rid of garden snails, you can offer a disposal service!
Cheers,
Mark.