Bamrumgrad Hospital and some of the other major hospitals in Bangkok have substantial dental clinics that will do teeth whitening and all sorts of other services. Charges at those places will run up around 15,000THB, cheaper than in the US but not exactly a bargain. Freestanding dental clinics also do whitening. Silom Dental Clinic is a well-known one. There's also one on the top (fourth?) floor of Siam Center mall; I used that one, and it seemed to be popular with foreigners. Charges were 10,000THB for laser treatment. It was a fairly clean and relatively modern place, like you'd see in a dental office in the US ten or twelve years ago, but my treatment area, at least, did not have the elaborate (read: expensive) computers and plasma-screen displays dentists in the US now seem to fancy.

Be forewarned, in case you haven't had it done before. The laser treatment leaves a sometimes sharp tingling pain in your teeth, probably by activating the nerves. You might schedule your facial or a massage for shortly after your treatment. I did that. It was something to look forward to while they were going to town on my teeth.