HUH???...look it up

all you need is carbon dioxide , why would you pay for something that comes outa your own lungs

i dont know but i would think not.because when you go to put them in the green houses the gases could get out in to the atmosphere.

I don't know, but you could do a chemical reaction in a beaker that had these as side products and you could preform it under the greenhouse.

Those gasses are available. Airgas companies provide them to universeties and industry.
Check the internet.

I think that'd be quite a difficult feat. What you MIGHT want to try is somehow getting these gases (which will be difficult in itself. Carbon Dioxide is easy enough through Dry Ice, but methane and Nitrous Oxide might be a bit tricker).

I suggest you find as clear of a balloon or plastic bag as you can and try to store the gases in there. You'll never be able to airproof a greenhouse enough without a lot of difficulty.

From what I understand, you're testing the insulation properties of the various gases, so it should work in just about any container.

Edit: As for those suggesting that you breathe or fart into your various containers, that won't work. The human breath isn't 100% co2 and the human fart is a miniscule percentage of actual methane (not enough for good results).

Well, technically, yes. You can buy CO2 in a tank (along with many other gases), and you can release them into a greenhouse. It might have the same effect as it does on the earth. For water vapor, just boil enough water in your greenhouse for long enough. If you want N2o you can buy that in a tank too. Unfortunately, this is very expensive because n2o is harder to make and find than something like co2. As for methane, (and I know that this sounds really silly) have the gassiest kid in class pass some gas into your greenhouse. This might be harmful for obvious reasons. You may have to be careful with these gases as human beings are not designed to breath it in. This is mainly a problem if you are talking about a greenhouse that you could walk into. If you were to walk into a greenhouse filled with co2, there will be a danger of suffocation. Other than that, I'm sure that it's all possible. For co2, I'd check a local paintball shop. If there isn't one where you live, the next place to look would be a sporting good store. I don't know where to buy n2o, but I'd assume that you have to special order it. The same applies to methane. (Don't try the source for methane posted above, it was just a joke.) And water vapor, which is essentially steam, just boil some water and keep it boiling.