I took a class at Preston Art Center many years ago and can't recommend
it enough if you're serious enough about it to put in quite a bit of
time. While I doubt the same guy is still teaching there, I have taken
other classes there and they've all been quite accomplished in their
field and are good teachers. Even the owner is quite an accomplished
airbrush artist. In the end I gave up on airbrush, it requires an
incredible amount of practice to get good at. Make one mistake and
you've essentially trashed your piece, usually the only way to recover
is to start over. But when I was trying, I found the class to be very
helpful, as well as many artists on YouTube.