@Tristan *****
Sorry I can not help you there. When I was looking in Bangkok I used FB groups and seemed to find more owners there offering direct. I found mine that way and stayed over 3 years in same place. One other way is to wander around areas you like - there are always notices about, tied on lampposts etc. If you can take a Thai friend as often the notices are in Thai. If in an area you like and you know the condo buildings you like walk in and ask the Juristic Office - they often know what is up for rent.