I am a complete noob and have never worked with any of these materials for these particular applications, but I do know a thing or two about chemistry and cars so I thought I would suggest a source for CO2.
The complete combustion of any hydrocarbon results in heat, CO2 and H2O.
For example if gasoline was completely combusted you would have the following reaction:
Gasoline can be represented by octane C8H18. Its combustion, in the most straight-forward sense is to carbon dioxide and water.
2C8H18 + 25O2 ~> 16CO2 + 18H2O
Modern day vehicles do a fantastic job of completely burning gasoline, once they have reached operating temperature, so you could just pipe some in from your exhaust pipe.
There are a myriad of other way cool chemical reactions that would release CO2, but this is about as cheap. simple and readily available as you can get.
