Buy them at the automotive body shop supply stores.

I use the finest. Thing is this, a professional needs consistency, so without straining, how do you know what's in the cup? You don't.

Realistically, I cannot see painting an airplane with an airbrush. I have a "jam gun", actually an old bottom feed which I use for areas that don't require detail. I have an HVLP which I use for larger areas and for clearing.

One simple thing I keep in mind, is it's all the same but different. That is, it could be graphixs on a motorcycle tank or art on a model. The approach is the same.

Charles