Search won't get you far.
Use HMT and Halo PC (NOT CE) and extract a model from a map (play with it, that you can search for) Then using Milkshape, import it in (it's an .obj) And...
If you have 3dsmax:
Select all the faces (put select mode on faces and hit CTRL-A) and go to reverse vertex order (somewhere at the top, maybe under verticies) The export it all as a Autodesk 3ds file. The fire up 3dsmax, import it in, and scale it up to a good size (Merge the master cheif from the tutorial map (data\levels\test\tutorial\models\tutorial.max)). Use Mr.Search on these forums to figure out how to export it as a gearbox model.
I was able to do this:
If you don't and just feel like messing around with it or for Halo PC:
Do the same thing, mess with the verticies, don't create or delete any verticies, and when you're done, reverse the vertex order again (flipping it back) and export it and then import it using HMT. There are plenty of tutorials and topics on this here, but you're better off just messing around with it. And dear god, get rid of the trial. Buy it or whatever

, just don't use it. It's evil.