Many paintball shops fill with Nitrogen. It werqs well for our airguns. I used it for a couple of years before I got my Yong Heng. Prices vary wildly from area to area so if you can find a good place to fill for cheap it is a good thing to use.
Just wondering what kind of pressures you are getting with nitrogen? Most of the shops that fill for welding supplies only fill to 2,000 psi around here.