Not specifically an "AirPac" but using just a regular air tank is a pretty common way to have "Portable" air. They cost around $30 at Sears or Lowes. The biggest problem is that gas station air pressure is frequently not that high so you would be running back and forth quite a bit. 100-150 psi is probably about normal, I would guess.
I have a 6 gallon compressor, and it pressurizes to 140 psi. I've never actually timed it so I could be way off, but I suspect that at 15 psi painting pressure I'd get about an hour of actual spray time before the tank pressure went to zip.