Graduating from Krylon to the good stuff.

Got a Badger 350 on ebay for something like $35.  It's the one that Norrell's uses/recommends.

For external mix brushes, Walmart carries them in the toy section by the models.

For an internal mix, try a Hobby Store like Micheal's or Hobby Lobby.  Some people don't like the internal mix brushes but internal is all I use for refinishing weapons.  You have alot more control over the finish as it is applied as well as a much smoother finish.  Camo jobs are 100 times easier with a double action internal mix air brush.

I second ebay.  Everytime I check prices on a 2 stage badger you can almost always find one for 50 or lower.

Paasche makes a very good airbrush...you can get 'em at hobby stores.  You'll pay more for a Paashce than some of the others, but the quality difference is noticable.  

www.bearair.com/

Harbor freight: 5 bux !!! works like my 40 bux badger :)

Quoted: Harbor freight: 5 bux !!! works like my 40 bux badger :)

Ditto, and to add,
Using the units plastic cup feed assembly off the one supplied, turn a small testers paint bottle into your supply bottle (remember to add the top air feed hole), and drop by a plumbing supply shop and get a few need O rings for replacement parts.  

The reason for these two items is that when using the canted bottom tester glass bottle (read with the lower section set up forward), you can get all the paint out instead of wasting the last little bit, and the glass bottle/new o rings is that you will cleaning the rig with acetone, and both the old plastic supply bottle and needle O rings will not last long due to the cleaning solvent.

The really funny thing about the cheap air brush is I picked it as a throw away unit for a single job, and now two years later, gallons of paint, it's still working fine.

To add, when you at harbor freight, don't forget to pick a simple in line dehumidifier (like $3), and the needed fittings to adapt the airbrush to your air compressor (read trying to use can'd air sucks since the can try’s to freeze up really fast if you don't float it in a bucket of hot water).

As for applying finishes, just remember two rules.  High air pressure/low flow will create a rougher finish (like anodizing), while higher feeds and lower air pressure will produce a smoother finish).  Best if you are going to do the testers feed bottle conversion is to use the testers paint to get a feel for the brush on a piece of paper (thin the paint down about 2/1 with acetone for the feel of Moly, about 3/1 for the feel of Duracoat (just use acetone to thin the testers paint since you will have a quart around for clean up anyway).

Hope this helps when you spend that whole $15 buy and set up the rig.