you can buy spray on engine paint from most auto parts stores.... here its about 8 bucks a can, more depending on the heat you plan to be running. the last can of paint i bought (chrome, perfect reflector) handled up to 800 degrees, and that wasnt the hottest i could get.
if using spray on paint, do several thin coats over a primer coat. you absolutely do not want a shoddy job that will eventually peel and flake into your tank. i would assume this is the same with any paint, not just spray.