Plants breathe carbon dioxide, so intakes sense to grow them in an environment that provides as much of that necessary gas as possible. Plants that are grown in an enclosed CO2-rich atmosphere have demonstrated remarkably accelerated growth---you can see the effect right away---kind of like accelerating healing in medical patients by feeding pure oxygen into their lungs.
Try to locate your growing room next to a gas appliance whenever possible to take advantage of carbon dioxide created by combustion in gas hot water heaters and furnaces. Pot-growing experts generally agree that 70 degrees F is about ideal for growing cannabis, and gas-burning appliances help to keep plants warm on cold nights, while traces of heavier CO2 that escapes being exhausted with more dangerous gases can be used to help your plants grow faster.
Probably the best CO2 generator is a simple low-pressure tank that would ordinarily be used to put carbonation into soft drinks, pressurize beer kegs, or power paint- ball guns. They range in volume from 20 ounces to more than 20 pounds, with prices ranging from about $25 for the former to about $100 for the latter. Some growers like to slowly bleed their tanks from a hose or hoses suspended directly over their plants, keeping them constantly refreshed with a flow of pure CO2. Others prefer to semiseal their grow room entrances with a plastic shower curtain and flood the growing space with carbon dioxide for about twenty seconds (presuming a small space of 4x6 feet or so) before sealing it for the next couple of hours. This hyperbaric chamber for plants really helps to accelerate growth and at the same time makes your plants unpleasant places for spider mites and other oxygen-breathing pests.
CO2 can also be generated by chemical processes. One of the simplest is to burn a safely enclosed (glass-jarred) candle inside your sealed grow room. So long as the candle is burning, it produces carbon dioxide, and little else, as the result of combustion. If the candle goes out before being consumed (unlikely), the atmosphere inside is saturated with carbon dioxide.
Another simple CO2 generations apostle of ordinary vinegar that has been suspended (duct-taped) upside down at a height of about one foot over a bowl filled with plain baking soda. A small (wire brad-size) hole in the center of the vinegar bottle's cap permits---ideally---about one drop a minute to fall into the soda below, where acid and caustic react mildly to produce carbon dioxide gas.