A local scuba store should be able to refill it for u for a couple of quid every time it runs out. U can probably find a local one in the yellow pages.
Just for reference (on average this is, obviously some guns use more air than others depending on setup etc.):
A 4500 psi tank will deliver approx. 15 shots per cubic inch.
i.e. A 68ci tank = 1020 shots
An 88ci tank = 1320 shots
A 114ci tank = 1710 shots
A 3000 psi tank will get you approx. 10 shots per ci.
i.e. A 68ci tank = 680 shots
An 88ci tank = 880 shots
A 114ci tank = 1140 shots
Using that you can probably figure out how big a scuba tank you may need, depending on how much u shoot and how often u need to refill ure tank.
Hope that's helpful