You have to get the carbon from somewhere... it's a matter of physics... matter cannot be created or destroyed, only transformed.
So potential sources for Carbon are:
Fossil fuels... hydrocarbon chains... must be oxidized into their component bits by the process of combustion in the presence of oxygen (to get CO2... that's where the O2 comes from)
Sugars... also carbon molecules, and you can break them apart using the fermentaion process by which YEASTS convert the sugars into CO2, among other things.... that's how the bubbles get into champagne.
Combustion of wood, etc... very inefficient and impractical, but you get CO2 as a byproduct of that too.

So anyway in my first reply I mentioned a CO2 generator that runs on LP, liquid propane, like for a BBQ grill... propane + atmospheric oxygen >>>oxidation/combustion>>> CO2 + water.