Tonic water will help....but only to a small degree...the C02 will dissipate quickly, so to really get enough to boost growth you'd have to mist them constantly (which isn't prudent, especially when flowering due to risk of mold)
Cheapest way to generate C02, the sugar/yeast mix...this is cheap, and will keep the C02 levels up for a couple of weeks, rather than just a few minutes (but depending on the size of your room, you might need to make a pretty hefty yeast res, like something the size of a 5 gal gas tank)