The Khao San Road travel agents can get you good deals. I booked a BKK-HKG o/w on Air China? last year for about 120US. I can't remember the name of the agency I used, but here's a link that lists several of the agencies...

http://www.khaosanroad.com/travel.html

P.S. found that link from the www.lonelyplanet.com forums, a useful source for budget travel info. You might find reviews of individual agents there.

P.P.S. You should probably expect a small credit charge surcharge on tickets purchased, seems to be the way of the world with these bucket shops.