I don't know enough about them to comment. Ambient CO2 levels run in the high 300s to start with, so at most you want to push that up another 1,000ppm or so, and mammal life is really good at pumping that up cheaply and safely under normal conditions. Grow rooms aren't usually the best enviroment for them, but fungal life also tends to generate CO2, so yeast is an option, start a culture and it will go until it either runs out of food (like sugar) or the concentration of alchohol kills it off (beer takes advantage of both properties). Or go to a Startbucks and ask for spent coffee grounds and start mushrooms in it.

There are chemicals that produce it as part of a reaction, a lot of things give it off when burnt, and then there is also bottled CO2 gas available.