I have seen people who are doing small grows use a simple method for CO2. You take a liter soda bottle (plastic), drill a small hole in the twist on lid, fill the bottle with water, add 4 teaspoons of activated yeast (common bakers yeast) and a couple of tablespoons of white sugar. Put the lid back on and set this above your plants. You must check this every few days by shaking the bottle with your finger over the hole in the twist off, if you get a build up of pressure, it is still producing CO2, if not, it needs to be refreshed by dumping half of the mixture, add fresh water and a couple of tablespoons of sugar again.
You would think that this alone wouldn't make much of a difference, but I've found that you will increase your yield by about 1/4 by using this technique continuously throughout the grow.
Found this in a post sometime back and kept it for my cheat sheet, hope it helps you out.