The name most synonymous with Thai silk is Jim Thompson. The American is credited with reviving the "dying" art of weaving Thai silk in the years following World War II. He sought out weavers in the northeast of Thailand - where most of the silk is still sourced - and made sure the craft was passed on. He advised on what patterns and colors would sell, and of course he purchased a lot of final product for his own shop in Bangkok.
Over the years following Jim Thompson's disappearance, the Jim Thompson Thai Silk Company has grown and expanded to become not only 'the' name in Thai silk, but a complete 'lifestyle' brand exemplifying modern Thai style. Not only are there clothes on sale, but upholstery fabrics, wall coverings, pillows, sheets, travel bags and sundry other items. Plus there's also a small chain of Jim Thompson Cafés around Bangkok as well as some other countries.
While Jim Thompson has a well-earned reputation for offering the finest Thai silk, such quality doesn't come cheap. You will easily pay three times more for silk items carrying the Jim Thompson logo than you will for similar items from other shops, although of course the quality may not be the same. Shop around a bit to help you decide if the quality is worth the premium price.
It's also worth noting that Jim Thompson is not a tailor shop. While the main branch does have some dress-making capabilities, this is not the place to go if you want a suit or other clothes tailor made. It's strictly off-the-rack at Jim Thompson.