Yes mate it is a dive tank you want and a paintball fill rig (see planets website) Dive tanks come in a wide variety of sizes but your best of getting a 15L dive cylinder, these range from £120-150 depending on where you get them from. An air refil normally goes for about £2-3.50 depending on where you get it filled at. A full 15L cylinder (232bar) will give you a decent amount of fills (maybe 7ish decent fills of a 1.1) but if you have a few people at training you will need a few cylinders
you could also go for the 300 bar option, only problem you would find is that if you just turn it on and ignore the pressure gauge you would prob blow your arm and left nut off if your bottles only 3000psi, what a mate does using mine is just stops it short of 3000psi, no troubles that way and you will get a fair few 3000psi fills of it also.
First time you ever use it you just set it to the pressure you want. Then it doesn't matter what pressure is going into it, it will never fill anything past what you have previous set.
Obviusly, they are bigger and cost more than the normal ones.
Only a suggestion, but if you have a 300bar dive tank, but only a 3000psi air system (about 220bar?), then ask you dive shop to only fill your tank to 220bar to remove any chance of overfilling your air system.