Grow Marijuana FAQ, Cannabis cultivation - marijuana growing tips & photos
That is a easy way to make CO2. Basically anything with yeast and sugar (toast helps).

I have a 2 liter soda bottle that I fill up to where it curves inwards with warm water, dissolve a cup of sugar in it, wait for it to cool down to room temp, then put some crumpled up burnt toast (half a slice or so) and then I put 1/2 tablespoon of yeast in there. Then I got the cap and made a small hole with a knife in the top so I could barely get some 1/4" aquarium tubing in, put some sealent on it. Measure out how much tubing you want, cut it, then take the tube outside, light then end you just cut on fire for a few seconds then use some pliers to clamp down the molten plastic to create a end cap. Then make holes the size of the end of a pen where the tube goes over each plant and screw it all together and you are done!