If your gun runs on HPA (high pressure air/nitrogen), you can get a bulk tank from a scuba shop and a fill station adapter to fill your own tanks. It's a lot of hassel and you're better off just doing it at the field, in my opinion.
For CO2, you can buy/rent bulk tanks from a welding shop. You need one with a drop tube to siphon liquid out of it. The take will need to be hydro tested if it's out of date (same with the SCUBA tank). You also need a scale to make sure you fill the tank to the right amount.
It's a lot of hassel and isn't really worth it, unless you're playing out in a field with a bunch of guys who will chip in on the cost. CO2 fills at the field aren't THAT expensive, invest in a good regulator (like a Palmer Stabilizer - 800 shots from a 20oz) if you don't play that regularly.