The fact that you say that your local site won't fill air leads me to assume that you're just playing as a walk on, in which case how often do you play? Weigh the cost of Co2 fills against the cost of owning you're own tank..
If you play regularly and are thinking of joing a team then a tank might be the answer, but if you're going to joing a team then most have air tanks for you to use (subject to a contribuition to refills I guess)..
Alternatively - is there another site you can go to for an air fill?
In terms of running the best you can - does the Bushmaster 2000 have a vertical feed? If so you might want to consider buying an exspansion bottle (well thats what they called them inmy day..) as well as an anti-syphon tube, which is a much smaller bottle that feeds dierctly into the gun, whilst the 20oz feeds the exspansion bottle either via a remote hose or donkey (again.. thats what they called them in my day..) - the bit that the bottle screws into!
