Catalyzed urethanes are easy to find, and every one I know of is water white (non-yellowing). House Of Kolor is very well known, but pricey. Kustom Kolors is cheaper and only a 2 part mix instead of 3 part. PPG has several lines of product, I've used the PPG ShopLine which was decent stuff and is pretty much the same thing as Kustom Kolors. Dupont has catalyzed poly as well as Imron. The trick is finding a local autobody supply shop or hobby shop that specializes in airbrush work. If not you are going to have to order online.
The hard part is going to be applying a proper finish to a guitar with an airbrush. Most catalyzed urethanes are not recomended to be thinned to the extent that an airbrush needs. Which means your atomization will be poor and you'll get a lot of orange peal. The second problem is that your coats are going to be very thin and for decent protection you'll end up having to spray a lot.
Airbrushes are good for graphics work and sunbursts but when it comes to a nice finish, they fall short.
Also, read up in some of the threads on here about the hazards of catalyzed finishes. They contain isocyantes which are not blocked by any respirator, even the charcoal filters, they are also absorbed through the skin.