I visited Thailand many times. Unfortunatelly I wasn't lucky and have not find any more place to buy some bonsai trees but Chatuchak Market. This is Bangkok's largest market and is located not far from Mo Chit Sky train station. If I am not mistaken flower market is open on Tuesday and Thursday, on weekends it's just dress and so on market.
Most of the trees you can see there are in spiral and heart style
If you are interesting in suseiki I highly recommend you to visit
Thai Rare Stone Museum - Bangkok 1048-1054 New Road, Bangrak, Bangkok -it is not far from city center
http://www.rarestonemuseum.com/english/index_en.htm
You can find some information about bonsai in Thailand here:
http://www.geocities.com/Tokyo/Pala...4/thailand.html or
http://www.geocities.com/bonsai-in-asia/thailand.html