what air tanks are you using? scuba tanks are around 3000psi (232bar), you can pick a 12 litre up (in test) for about £60-£70 though I recmmend a diving website for more choice, a fill rig so that you can top your bottle from the tank is £45+
Air fills at my local dive shop ar £1.50 but they can be as much as £3. I used my 12 which kept my 0.8 and my mates 1.5 running for well over a box of paint