I recommend Raja's on Sukummvit in BKK. They are about three blocks walk from the Westin Sukummvit or Sheraton. Walk out of the Westin and turn right and you'll run into them down the way.

I have pieces both from their fabric and from fabrics I brought with me. Both are wearing well. The key (I think) is knowing something about fabrics and picking as you go. The pieces I had made from the fabrics I either brought or handpicked have done well. Pieces I have ordered by mail or had a friend pick up (where I knew he didn't know much about fabric) haven't pleased me as much.

As far as buying fabrics in the states, I recommend Britex Fabrics on Union Square in SFO. There are also several good shops in NYC in the Garment District but I don't know them by name... ask a good tailor for recommendations there.