Master Flow makes a great/cheap way for you to have On Board Air. The MF-1050 is the most popular one and can be found at almost any auto parts store.
I recently went to Costco on 3/1/07, and found a MF-1050 for 49.99. This is a great deal and comes with everything you see here. The reason I bought it at Costco was because of the extremely generous return policy they have.
Here are some specs:
Motor: 12-Volt DC
Fuse: 30 Amps
Maximum working pressure: 120 psi
Maximum re-start pressure: 150 psi
Maximum recommended continuous operating time: 40 minutes
Recommended on/off cycle time: 15/30 minutes
It also has a very nice bag and other adapters you can use for different applications.