Hi and welcome to SalonGeek.com
I used an airbrush for many years and loved it but to make money at it in the nail business you will have to do allot of nail art to recoup your investment and you will need to have it set up all the time and ready to go to make it work practically for you.
Any good acrylic airbrush paint will work for you from an art store. I used Aztek paint because it was superb, but I don't know where you would buy that now as airbrushing is now rare on nails with the advent of super-quick and easy veneers and appliqués like Dashing Diva & Minx and super-quick ombré and colour fading using pigments and stamping instead of stencilling with an airbrush .. All so much faster, cleaner and easier and longer lasting, not to mention cheaper and more popular, than setting up an airbrush station. If you are a professional nail technician, I'm kind of surprised you have not considered those things first.