In my opinion, I would go yo Home Depot (or Lowes) and get a "Husky" air compressor (1 gallon tank, 135psi max) for $99 (or something similar). Since the majority of air brushes use 25 - 30 psi for painting, this will provide you with about 10 - 15 minutes of non stop painting. (I doubt you will be painting much over 5 minutes non stop).

Couple this with a DECENT 2 stage air brush and you have a system that will last a lifetime plus I am certain you will find other uses for the compressor (car tires, kid's toys, etc)