Canned co2 or pressurized co2 can be found at places that do co2 refills in your area you can find tanks craigslist also. Those co2 that are used in personal soda machines are cartridges, from what I know there isnt a an adaptor for co2 cartridges. Maybe fluval had one
Do a paintball co2 setup. Paintball co2 tanks can be had for cheap.
Don't forget you have to get a regulator. If you want to do paintball co2, then there's a guy on PLANTEDTANK that has regulators for paintball co2 for a more than reasonable price.