Here is
more info on HMMWV types.For a kit, it depends on which type you want to build. Tamiya's is the best on the market. They have both a standard M1025 weapons carrier, and an M1046 TOW HMWWV.
To make the M1114 Up-Armored HMMWV, which is the most common in Iraq right now and probably what you saw, you have to also get a conversion set from Mig Productions.
Academy makes pretty much every other version of HMMWV, but they are a little less detailed than the Tamiya ones.
For other accessories and conversions, check out Blast Models out of France, CMK, Verlinden, Pro Art, and a few others.