This agency was recommended to me by a good friend who has lived here for 20 years, live used then myself and have a lot of faith in them.
Caveat: you'll need a Bangkok Bank transaction account and if you don't have one they can get you one on the spot at the nearest BBL branch for ฿4,700. Most of that is to pay for a fairly limited insurance policy. It's a bit steep but that's how it works, the magic doesn't happen by itself.