There are filling stations out on the field, I think they're really just scuba tanks for compressed air. They can also refill your Co2 tanks for you. If you head to any paintball shop they should be able to fill up your tank for like $2.00.
Usually any marker that can run on C02 tanks and has the right threads, you can run compressed air with. The really cheap markers from Zellers with the 12 grams, probably not unless you get a drop-forward installed or modify it so you can screw on 12/20oz C02 tanks.
I wanted to move to compressed air but I don't think it was worthwhile on my electro marker (Jt 6.0) for now. I used to run an old Piranha, if I find a good deal on an autococker (yeah I said it!) I'd move to compressed air.