I use sugar/yeast CO2 generation in a vented cabinet with great results. You can buy baker's/brewer's yeast (active dry yeast) at nearly any grocery store and the cheapest sugar you can find is fine. One pound of sugar will produce approximately 1/2 lb of CO2. (the math gets a little complicated here, "pound" is not a measure of volume- 1 cuft of CO2 = 8.75lbs at "standard pressure") Make sure you start with warm water (not hot) then dissolve your sugar in the water, add the yeast and a small amount of baking soda to start the process (baking soda not required, just faster). If your room is vented you'll need to take that air exchange into account when you calculate how much sugar it's going to take to produce the amount of CO2 you need. (assuming you haven't followed the extreme air exchange "guidelines" many recommend) Multiply your cubic footage by .0015 to determine how much CO2 you need to introduce to reach roughly 1500ppm, I needed four one gallon containers to achieve an appropriate concentration. (Outdoor air is ~380ppm CO2, indoor is ~600ppm) Even a nominal increase in CO2 concentration is beneficial, realizing 1500-2000ppm will increase growth rate and yield dramatically. Add an air lock to capture the alcohol (by-product of the fermentation), you can buy a cheap plastic one at any brewers supply or make your own from another empty container. The recipe that works best for me is: for each gallon jug fill 3/4 full with warm water (I usually fill it half way then finish filling after I add the sugar), add 4 1/2 cups of sugar (2 cups = ~ 1 lb) and mix, add one packet of yeast (~2 1/4 tsp) then 1/4 tsp baking soda. Four 1 gallon jugs prepared this way deliver a substantial volume of CO2 (about .18 cuft/hr) and last roughly 10 days. There are a ton of "how-to's" on the web that show you how to build it or I'll be happy to help if needed.
That's a "quick & dirty" summary, please let me know if you have questions. I've attached a pic I found somewhere that illustrates the concept, the "40% growth increase" is not my claim, all credit to the original poster.
