Thailand forum

there are different types of Thai dance...places around Bangkok & Chiangmai have performances geared towards tourists, usually part of cultural shows or dinner shows in restaurants (e.g. Rose Garden, Siam Niraimit http://www.siamniramit.com/)...they tend to be short items, a quick showcase of a few different dance styles (most tourists wouldn't sit through hours of it )

if you are really into Thai dance & want to see a full performance of khon in Bangkok, try SalaChalermkrung theatre (in Rattanakosin area somewhere between Wat Suthat& Pahurat market): http://www.salachalermkrung.com/
more about khon: http://www.mahidol.ac.th/thailand/khon.html
could also check out Patravadi Theatre: http://www.patravaditheatre.com/ (esp if you are interested in dance training/techniques)
there's also the more 'experimental' www.pklifework.com but they seem to perform mostly outside of Thailand ('tis the reality of traditional Thai dance...lack of support within Thailand)