I also use an ultrasonic cleaner on my airbrushes.
I use a jewelry cleaning powder that came with one of my ultrasonic cleaners. I'm not sure this is the exact product I use, but it is the same idea and this is where I will probably go to buy when I run out.
FWIW, as long as I clean the airbrush immediately at the end of a session, I don't really use the ultrasonic cleaner that much. But it can help with stuck on messes.