I find this site very helpful in finding most things long term rental wise anywhere in Thailand. You might find it useful for a serviced appt. Select your region first, then select what you are looking for, then select your price range. For your case select Bangkok (central region), then rentals, then the bt20000 -bt40000 price range. There are a lot of options to chose from.

BahtSOLD.com | Thailand Classified Ads | Ads for 18th May 2007 | Page 1