The other idea that has been crossing my mind is building my own compressor considering there really isn't that much to them. I could use an old propane tank for the tank.
Well, I've got a pump that has been sitting in the loft that was given to me about 4 years ago to convert for just such a project. That never happened so it's yours if you want it.
I've also done the tandom setup like mike mentioned, biggest drawback I recon is floorspace used.
Alc also mentioned a viable option, and a lot of them (like the IR T-30 my shop uses) will already have the unloader built in. Ours is a 25 hp and the moter runs continuasly with the pump kicking in as needed so an engine conversion would just be a matter of mounting and pulleys (you should see the v-belt that monster uses). The only thing I can think of that might be an issue is, your gonna need to consider the differances between electric and internal combustion. An electric moter is always at 100% torque, and pump duty moters have big capaciters to handle load surges. An engine has a curve, so to end my babble, if you find a nice little 3 ph cheap that has a 10 hp moter on it, you might need to put a 15 hp engine on it for the conversion, meby less if you go diesel.