I have heard of people using baking soda with a vinegar drip to add co2. I personally have never tried this, as some carbonated water on the foliage or the method OM talked about are nice options.

You have to be very careful with that gas as it is fatal to humans and pets if used incorrectly or without excellent ventilation and supervisor. (Not for the noob IMHO, as it won't increase a small grows yields that dramatically to risk something that could prove dangerous.)