Sadly I'm not in Bangkok, I currently live in Hanoi, i was just in Bangkok for a long weekend.
I actually sold my 2 Mamiya 6s in January, but I do have a Voigtlander Bessa 667w if you end up in Hanoi.
Photogallery may be able to help you, there is also a shop in MBK shopping mall that sometimes has Medium format gear.
Theres a very good blog post about film photography in Bangkok/Thailand by someone who is either an RFF or APUG member, but I can't remember who it was at the moment.