Well you have the most reliable method of getting co2, from a tank with a regulator and a timer. This will allow you to control the amount in the room, for how long, and when.
There is a product out called co2 boost, it comes in a bucket, I have heard it works well, but it is a bit pricy at 150 a bucket, which lasts about 3 months.
There are co2 generators that I believe may run off propane, but they create alot of heat, and maybe humidity as well.
So thats a few options, I've heard the yeast doesn't really work well, but that only from a few people that I've discussed it with, so maybe others have had good results. I tank is deffinitely the most reliable source, with the most control.