More details, a few months later...
To everyone who responded to my first post,
I can't believe I left out important information..sorry about that! Here are some clarifying points:
-I'm living in DC, so looking for a flight from this side of the US. I am ok with traveling a bit to get a cheaper flight, like NY or Boston. That's pretty much the furthest I can go though.
-I need to wait to travel until after June 17 next year, because of Ben's schedule with Peace Corps. I would consider going in the fall next year, which it seems may be the best bet, but I would prefer to go some time late June-September. I'm also going for 2-3 weeks.
-His placement is in southern Cambodia, very close to the border of Vietnam. Since he'll know the area and speak the language by then, he will meet me at the airport (PNH or HCMC) so I won't have to worry about overland travel. If I fly into Bangkok, which I will look into, I'll check out Air Asia for a flight closer from there.
Thanks for all the helpful responses, I've been checking out the websites (itasoftware.com) and (thaifly.com). The first actually found me a flight for $1000, which is the cheapest I've found out of the East Coast US. If anyone has more suggestions based on what I've elaborated on, that would be awesome. Thanks again! 