I dont really have any experience of bying a bike in thailand but i do know there are big import taxes on all foreign vehicles going into thailand, bar pick ups and commercial vehicles.... this means they tend to be expensive or illegally imported in bits.... also, thais are notorious for not carrying out regular maintenace on there bikes and so finding a good one may be difficult. However, there are a fair few proper bikes in the country and there is a good bike shop in Chang Mai run by a german guy who may be able to assist, details already posted in "Repair shop" section.
I would have thought it may be best to try and find a fellow traveller who wants to sell his bike in that region??
hope it helps, best of luck.
Some day so soon....