From my experience, you need a dive card to get SCUBA tanks filled. Besides, I admit - I'm lazy. I don't want to have to bother a fire department that will be willing to fill a tank for me just to shoot an air gun.
People are making booster pumps that work off of standard air compressor to get the pressure to a level needed to top off 3000 psi air cylinders. http://www.youtube.com/watch?v=HWoH7f90c4U
. I am aware of at least one company that produces a commercial booster pump. But, I'm cheap and lazy - this technology will get a bit more refined, and then I'll think about jumping in.