When my wife and I bought a Jeep I did a lot of research trying to find the best balance of quality vs. cost in buying equipment for off-pavement travel. The best little air compressor I was able to find was the Interdynamics Truck Air. It's not the most powerful, nor the cheapest. But it is a reliable unit for a reasonable price. I've seen them in local stores (such as K-Mart) for $39.99. The link above is provided as a reference.

Dennis (The Menace)