The Testors unit really is a paint sprayer and not a real airbrush, that's why it doesn't work that well. Any brand name airbrushes within your budget should work fine - Badger, Paasche, etc. Single action ones are easiest to use and clean, but double action models can do finer work. Either type will be better than what you have.
And as you've found, the propellant cans aren't very efficient and are costly. A compressor can be initially an expense, but will improve things considerably. Look for them at home improvement stores, Wal-Mart, etc., and select one with a tank. They can be had for under a hundred dollars.
As a bonus they can be used for other things around the house - inflating tires, pools, balls, etc., and some can run brad nailers. Mine came with a brad nailer and I've only used it a few times. But our eldest son borrowed it recently and it worked great fastening some new MDF baseboard in a room he was working on. It's not a nail gun, not that powerful, but great for trim work.