There are many popluar markets and if you are going to barter at any of them, you are advised to produce the exact amount of money you are prepared to pay e.g. 300 Thai Bahts. If you have the correct sum, you'll have a bigger chance of the seller agreeing. Try the local restaurants where Thai people eat, even if comfort may be lacking, the food is very good and cheap.
Change to Thai Bahts before you leave and do ask for small denominations for the first day for things like taxi rides and bartering.