Obviously, any exhalation would still be mostly nitrogen gas, close to what you breathe in (78% in atmosphere).
I HOPE you're just suggesting the ballon thing as a joke, either that or you should abstain a bit before posting.
A simple one gallon fermenting container will provide a slow continuous supply of CO2 for 3-4 weeks. Not a great solution, but not bad if you just have cirrculation fan in a small area and no venting fan to suck out extra CO2 along with heat. No screwing with ballons with pinholes that may last all of a few minutes, if that long.
One recipe:
One gallon jug (from Arizona Tea for example). 3c sucrose (table sugar) boiled in 5c H2O for 5min to help break disaccharide into component glucose/fructose (easier for yeast to munch), another 5c cold water added to boiled sugar solution (cold to speed cooling off), 2c apple/grape juice (or similar) to provide acidity and protiens for the yeast, and another 2c room temp H2O with yeast started. After yeast has had a chance to hydrate (30min?) add it in, IF solution's cooled down enough. Temp shock kills yeast. Add a bubbler to watch CO2 production if you want (airline tubing from fermenter sealed with silicone sealer -> submerged into smaller bottle filled 1/2 with water). Replace as CO2 bubbles slow significantly. Not sure what's a good amount, but probable depends on room size/# of plants. Add more fermenters and stagger deployment (ie. 3 gallons, replace oldest one each week) to keep CO2 levels even.
Airline tubing, tube connectors, silicone sealant/sealer can all be found with aquarium supplies at local store like walmart.
As a bonus, take extra care to rack (remove liquid, leaving dregs) the fermented brew after 3-4 weeks (and rack again, and again) and you'll have a drinkable 'wine' in a couple months too. If you want to try this, you also need to take care to sanitize the frementer/racking container prior to use.
More info and example can be found by searching for DIY co2 (for aquariums).