Sometimes it's much more expensive to find that cheaper method. The only
reasonable alternative to compressed gas cylinders is using dry ice, but you
may have difficulty controlling the flow rate of the gas.
Just go buy a cylinder. CO2 is VERY useful - not just for welding. You can
make sand molds with sodium silicate and all types of other crap. It's also
great for putting out fires, cryogenics, gas powered pumpkin canon, bla bla