If you do a google search for Vietnam Travel Agents, you'll see quite a lot of sites. have a trawl around the websites and see if anyone has what you're after.
If not, contact some of them direct.

We used Tonkin Travel ( found through a google search). They are based in Hanoi but have agents in HCM.
Or you can google Sinh Travel - they are in HCM and offer a large range of budget tours, flights and travel packages.