I've used Chawla Travel many times and they are very reputable and very experienced in dealing with heavy flyers.
Ask for Rani, she is the daughter in law of the owner and founder. They DHL tickets to you anywhere in the world.
In HK, you can try Tiglion travel (www.tiglion.com) and ask for Raina (names are coincidentally similar).
Like everyone else has said here, I'll add that you should depart from BKK for your side trips and you'll save 30-50% than if you departed from HKG. I always save a coupon to get back to BKK from HKG to take advantage of the fare differential.