I was unaware of the possible problems and had heard of fire departments willing to help out. When I buy my first PCP I plan on doing just what you suggest. Substantial initial outlay to achieve what I seek. Close to $5000 with scope and compressor and pellets.The problem is most tanks dont have a standard nitrogen valve/connector so the breathing air compressors wont connect. And why would a fire dept bother to fill a tank? Its not their job. Filling high pressure tanks is not without risk.
Buy an PCP air compressor and DIY. Buy once cry once.