Propellant cans?

Propellent cans are terrible, expensive and hard to regulate, all which has been said before.

 You can buy a 5 gallon air tank at Harbor Freight or similar place for $20-30 and refill at your local gas station. Or go to a tire store and buy the biggest truck tire they have and get an adapter for the tire stem (available at Dixie Art or Bear Air, maybe even your LHS). That could cost as much as the air tank, though.

If you have a garage, basement or aren't worried about noise you can buy small "garage" style air compressors for pretty cheap.

I have one very similar to the one I posted and it works pretty well, though it is LOUD! I keep mine outside in a rain proof and ventilated box and run a hose through my basement wall. I can barely hear it from there, but I am careful about not using it late at night or too early in the AM.

edit: poking around the Harbor Freight website I found this, which is a pretty good deal for a compressor! /edit

As mentioned above, propellant cans are evil, but in a pinch, you can get by.

They are problematic in that they are difficult to regulate, and it's impossible to maintain a consistent pressure.  

As the pressurised gas escapes from the can (ie. through your airbrush in normal use) it chills the remaining gas in the can (under the same principle as the condenser in your refrigerator). As the temperature drops, it becomes more difficult for the gas to "boil" out of solution from the liquid portion of the propellant. The net result is that the deliverable pressure to your airbrush will drop, eventually reaching the point where the airflow is insufficient to draw paint through the airbrush (you need to learn to recognise when this happens, as it's pointless to try to continue - if you do, you're only wasting propellant). At this point you must stop painting and wait for the can to return to room temp (restoring air pressure). To speed this process, you can stand the can in warm (not hot) water.

BTW: Here in Australia, large size propel cans run to around $25 (Aud) per can.

4 cans = 1 workshop compressor 
6 cans = 1 "hobby" airbrush compressor

Propellant cans are evil.  They get cold as you use them, and are don't really provide a good regulated pressure.  On the other hand, they beat a poke in the eye.  If your financial considerations put you into the choice of painting your P-38 with a Paasche H and propellant can or staying with hand paint brushes, definitely get the airbrush now, and upgrade to a compressor as soon as feasible.  For the first few kits that I airbrushed, I used propellant cans and the results were light years ahead of what I could do with a hand paint brush.

As to how long the cans last, maybe a two or three kits.  I kind of depends on how you paint and weather.  As I recall, the large cans go for about about ten bucks.