Hi we spent a month in Thailand and the best place we found was Chatuchak Weekend Market in Bangkok.
It would take a life time to see all the items they had but we bought copy diesel trainers for £2.50 a pair, which I am still wearing 18 months later.
We bought lovely combat shorts and trousers for £3 per pair ans copy reef flip flops for about £2.
We haggled really hard and just walk away they soon come running.
We took one ruksack between two on the way out and bought a new one there and filled it!!
To get here take the Skyline for about 20p.
We found that in Phuket they were less willing to bartgain and more expensive.
Much of what you buy at home is actually made in Thailand in the first place so i would have no concerns over buying clothing at all at the end of the day a tshirt is a tshirt wherever you are.
We also bought additional digital camera memory from a Kodak retailer this was much much cheaper than England and we have had not problems at all.