Benjamin High Pressure Hand Pump
I got this with the intention of filling small paintball compressed air tanks and it's working great for that. It doesn't require any kind of adapter for the fill valve that is on standard compressed air tanks. Some simple assembly is required. Pumping this thing is not easy requires either brute strength (100 lbs or more) or proper technique (utilizing your body weight) once you get above around 1000psi. Also it will take hundreds of pumps to fill even a small tank. My 13ci went from completely empty (0psi) to full (3000psi) in roughly 250 pumps, and that's at roughly 15-20 pumps per minute. So you can figure that a 48ci tank will take roughly 950 pumps and a 68ci 1300 pumps. If you intend to use this to fill larger tanks like the 48 or 68, don't expect to be able to do it in one night unless you are willing to spend 1-2 hours and in really good shape.
It only fills to 3600psi. It does have a pressure release so you don't blow out the O-rings.