Do not use dry ice, it is very costly and time consuming (unless you live right next to a place that sells it at a discount).
I have attempted to use yeast and it is also costly in the long run and there is maintenance.
My only suggestion is that you save your money until you have enough to actually buy the proper equipment designed to dispense CO2, the initial cost will be high, but the long term cost will be cheaper.
On a side note, I would not even waste the time or money adding CO2 unless you have the essentials MASTERED and down to a TEE. Light, humidity, air circulation, PH, watering, etc.
More often than not, the money and time invested in CO2 would be better spent investing in better equipment used to facilitate the mastery of the essentials (or security?).