I have been studying Muay Thai for several months, both in the Bay Area and on a retreat to Thailand. I practice at Fairtex, both here and in Thailand. Their trainers are previous Thai champions with a wealth of training experience (with foreigners and Thai fighters). They have gyms in San Francisco and in Mountain View, with expert staff at both. Fairtex is also a great place for beginners.
Check out their website for further information: http://fairtex.com/