It's important that you use common sense, and safety at all times. Have your hydrostatic testing done on their rountine basis!
I left the world of CO2 behind for a couple of reasons (cost and convenience) not to mention the better consistency I was told I'd get by going to HPA. So with that in mind I set out to get my own fill station.
First place I went was a Scuba diving shop and looked into the price of scuba tanks. I found both used (white one)and new (the yellow one)available at fair prices. With the purchase of a tank most shops will offer 1 free fill, I inquired about additional fills, the shop owner told me I could buy a punch card, that would allow me 20 fills. So I signed up.
Now "why two tanks?" you ask. Well what I've been told (and the method I've followed since) is that the first scuba tank fills the majority of your guns tank, and the second tank tops it off. That way the second tank uses less of its air. Once the first tanks has done about 8 fills (on a 68ci tank) it's pretty tired and ready to be refilled. So off to the scuba shop re-fill it and now it becomes the top off or #2 tank, and the other becomes the first fill.