Typical air brake compressor will be in the 5 to 10 cfm range. The bigger ones are in newer trucks and ones used to pull doubles and triples.
If you have some time, watch eBay and the local want ads for a used unit (or 2, if they are cheap). There is still the occasional good deal out there. I've got under $500 in by 7.5 HP 2-stage but it took a while to find.