This Health Land brand seems to be well regarded among Thais. Some of the original locations are out of the way and focused on Thai clientele so not sure about potential language issues, but they have expanded into the tourist zones.

http://healthlandspa.com/

Maybe you could say your preferred location(s), treatments, price range? Metro Bangkok is massive, and it might take an hour or two to get to a place via car/taxi (rush hour/inclement weather/flooding).

Most hotels have on-premises massage, or have arrangements with outside providers who are often in the building.