try mixing some yeast sugar and water in a bottle, the bubbles that makes are co2 also dissolving carbonates in acids makes it too, try indigestion tablets, bicarbonate of soda, stuff like that in vinegar or something. none smell right cracking, but they all make co2 alternatively you can get co2 canisters fairly cheap and there are places you can get them filled fairly cheap, someone told me if you set the release valve so that it takes 5-7 minutes to fill a balloon, and then its letting out a decent amount and should last 2 weeks or so.