Use http://www.travexnet.com. They are in Bangkok. I have used them numerous times and they are very reliable and, in my experience, offer good prices.

Remember that it can sometimes be cheaper to buy a round-trip ticket and throw away the unused return.

From BKK to HKG, they are showing a fare on Dragon Air, return for 4,850 (about US$125.00).

Tell them what you want and see what they come up with for you.