I lived in Thailand for five years recently, however it was before I picked up fly fishing. I gear fished around a small village where I have a house and pulled some weird fish out of rivers and ponds (the locals thought I was crazy, since they'd just throw in a net and pick out what they wanted to eat that night).
I'd sometimes ask someone what kind of fish I'd caught and I soon found out that Thai is not a real scientific language. Fish names translate to things like "long fish", or "black fish". Squid are called, "pla muuk", which just means, "ink fish".
So, while I don't have any good fly fishing info, I worked as a journalist in the tourist industry over there and I'd be happy to tell you anything you want to know about where to stay and how to get around.
Oh, and one more thing. Bangkok's got a decent population of sport fishers so there are some small shops selling gear (I can tell you where), but I never saw fly fishing supplies. You might want to bring your own. There's some crazy huge bugs over there...bring some flying ants and giant cockroaches. Tie your mosquitos as big as you like.