I had this done two years ago at Bangkok Smile in their Ploenchit Road clinic (opposite Robinsons Dept Store / British Embassy) - they have two other clinics, both in Sukhumvit.
I found them through Google and then asked the other forum users at the time for feedback and it was very positive, including a post from an American dental hygienist (or technician) who favoured this clinic over her own back home!
It cost me about A$320, which is roughly one third to a quarter of what it would have cost to get it done in Melbourne. At the time there was a special promotional offer of B1000 off if you could produce a copy of a magazine with their ad ... which I found sitting in their waiting room!
Clinic looked a little "humble" from the outiside - quite small and on a very busy main road with rather ramshackle drinks/good stalls either side, but inside it was immaculately clean and spacious.
I had an excellent Thai (female) dentist who had studied and trained in the US. Their equipment was state of the art and they were using the latest American techniques.
Can't say that the procedure was very enjoyable - but the result was good. I didn't want those brilliant Hollywood "tippex" white teeth, I just wanted my own teeth a couple of subtle shades lighter, and that's what I got.
I was travelling with a bunch of friends and deliberately arrived a day ahead so I could have this done and then recover and be ready to enjoy my holiday with them. Think I had the procedure at around 2:00 pm, and it took about 1.5 hours - then I went back to the hotel with my painkillers (which they gave me) and went to bed, feeling very sorry for myself. The first of my group of friends arrived at the hotel about midnight and I still felt a bit unwell, but by the time the rest arrived at 6:00 am the following day, I was 100% recovered.
So, just take that into account and don't think you can have it done in the morning and have then off for a full day of sightseeing or activities right after that! Obviously, everyone's recovery is different, but the pain did linger for quite a few hours and it's just sensible not to have any plans other than bed and TV!
I was impressed with Bangkok Smile and am happy to recommend it.
Web is
Dental Bangkok, Dentist Thailand by Bangkok Smile Dental Clinic in Thailand.
Good luck!
Cheers
Daniele