a) buy soda water, mist or spray on plant leaves. (mixer, tonic water. Make sure it's carbonated water ONLY, no salt etc)
b) drip vinegar into baking soda.
there is another one with water, sugar and yeast but you will have to look for that one. These are the cheapest solutions.
Other solutions might be to put a regulator on a paintball co2 bottle (or on a larger co2 canister that you can purchase from your local hardware store). Or to use co2 generator systems.