Umarex also makes a cool CO2 tank. Uses 2 12g CO2 cartridges. has the same threading as the 90g cartridge, and unlike the 90 g tanks it can be removed, and will hold its contents.

Respirator air tanks, from Scott at least will hold 3200 psi of air, or CO2. Sometimes they can be sourced cheaply. I had one filled with Nitrogen, with a regulator, and a Shrader chuk. Kept it in the trunk. I bought it with a fresh hydro test stamp, for $25 a fire equipment supply place. They had taken a bunch in on trade for the Kevlar tanks.