If you don't have a compressor there's always one of these:
$50 - http://www.harborfreight.com/cpi/cta...emnumber=91772
I had one similar. It may not spray thicker paint too well so thinning would be required. I broke the gun on mine partially my fault and partially because they are very cheap guns. This happened before I painted my bus. I have a big compressor so I used a standard spray gun. It's still an option if you don't have a compressor and want to spray your bus.
On my bus I used implement paint made by Valspar (usually sold at farm and fleet stores). It was about $20 a gallon and gives a nice glossy smooth finish when thinned and hardener added. It does have adhesion problems with bare metal.