I did a fair amount of 100% Thai silk fabric shopping (my wife is a crafter) while in BKK so I thought I'd post the information here for others who might do the same.

I visited the Jim Thompson main store on Surawong, Jim Thompson Outlet on Surawong, Maeliard Thai Silk in Old Siam Plaza, and Jim Thompson Outlet on Sukhumvit Soi 93.

Jim Thompson main store on Surawong - Wasn't here long, more of an Outlet kind of guy. Came here on accident while looking for the outlet. Cheapest fabric was around 490 THB per meter, I think 40" (1 meter) wide.

Jim Thompson Outlet on Surawong - Definitely cheaper than the main store. Cheapest fabric was around 380 THB per meter, 40" wide. Received detailed receipt with VAT refund possibility, didn't quite buy 2000 THB worth of stuff though.

Maeliard Thai Silk in Old Siam Plaza - This place was a pain to find. None of the guide maps (a real Bangkok street map would have been useful here, I later verified) show the streets in enough detail so I did a good amount of walking from Pier #5 before finding this place. Selection is pretty big, a lot of stuff is not in a bolt form or on rollers though. Looks/feels kind of like a lot of remnants. A lot of solid colors, few non-solid colors. Cheapest fabric was around 280 THB per meter, 40" wide. Storekeeper was also willing to sell in yards (90cm) for 260THB per yard. Got a very generic receipt with no hope of VAT refund.

Jim Thompson Outlet on Sukhumvit Soi 93 - I inadvertently saved the best for the last. The store isn't too far from On Nut station. I think it took me about 12 minutes to walk to it (9ish on Sukhumvit, and 3ish on Soi 93), I am a fairly fast walker. There is a cafe on-site but I didn't go to it. This was by far the biggest fabric store I went to in BKK and the prices were quite good. Carries cotton, cotton/silk blend, polyester, and polyester/silk blends in addition to 100% silk fabrics. I think the cheapest 100% silk fabric is 300 THB per meter at 40" wide. There is a huge selection of much nicer stuff as well, and a very large selection of wider fabrics (i.e. 55.5"). Cashier was very helpful and gave me the form to fill out for the VAT refund.

I also went to a few Jim Thompson stores and found the prices were the same at the different stores. It's nice to know that there isn't an additional mark up for a hotel location for example.