Thanks for that - bahtcoin look reasonable, I'll have to try for a small amount first and see how it goes
In addition to the above, several people above have mentioned localbitcoins. They seem to be active in Thailand, though it looks like you'd need to do a bank transfer (unless you met the seller face-to-face). I've had good results (in the UK) with localbitcoins and bank transfers, but obviously my experience in the UK may differ from yours in Thailand!
One issue people tend to have with localbitcoins (apart from the assumption that you can only trade face-to-face) is the wide price variation - right now I'm seeing anything from 18900 to 26800 THB per BTC. My advice here is to be patient and consider buying several small amounts instead of one large amount - which is probably good advice in general, since it spreads any counterparty risk. It might not suit your needs, though.
Edit: and, of course, Newar said exactly the same far more succintly and earlier
Ignore me, I'm a slow reader!