Pressure pot
I have used fancy rigs and modified fire extinguisher bottles. You can take a large fire extinguisher and have a machine shop weld a boss in there to thread an attachment for filling up with air.
Or paint pressure pots come with all the bits to make it work already:
http://m.harborfreight.com/2-1-2-half-gallon-pressure-paint-tank-66839.html?utm_referrer=direct%2Fnot%20provided