I don't use pot myself, but as a home brewer I have become more sympathetic to those who grow that funny little plant that raises such a fuss.
In any event, co2 is cheap and easy, it is the byproduct of fermentation. A 5 gallon batch of mead or wine can produce nearly 2 kg of co2 in about 2 or 3 weeks.
With some ingenuity, some computer fans, a heat exchanger (to avoid venting co2 and that ever telling skunky odour), you could likely keep a tiny grow closet well supplied with co2 just by starting a new batch of mead every few weeks.
Just some random thoughts here. If you want more info on making mead let me know. It isn't that hard to find a bee keeper who will trade a gallon of honey for a gallon of mead - reducing your brewing costs to near nothing, giving you nearly free co2 and providing you with a beverage fit for Odin.
Heather