All my co2 is made by yeast. But, I fill my paintball gun canisters at the local Wal-mart for pretty cheap. There's a sporting section in the back, might not be the same for all Wal-marts.
You could try going to a gun hobby store, they usually have paintball refilling tanks. Paintball shooting ranges usually have refilling stations that you can go to and fill free of charge.
Of course you would need to pay for rental or admission, but usually I just walk right in and fill up my tanks and leave.