I learned this after a friend got busted buy a canister of co2. It works so well I'd like to share it. White vinegar $1.87 per gal. 2Lb. baking soda $1.00. So $2.87 makes a bunch of CO2 I can't tell you how much . I was absent the day they taught us that in school.
You can use just a bucket. Pour in about an inch of vinegar into a clean bucket now throw in a few teaspoons of baking soda. BANG you have co2. It's really hard to give your plants to much co2. I go thru about a gal. of vinegar and two Lbs. of baking soda a week. In a 10 by 10 room the CO2 makes a big difference. Instead of a bucket you can get Fancy with a few milk jugs and aquarium tubing.