In the majority of US states you can order carbon dioxide testing online and have the blood drawn at a local blood draw center (see my general overview of the process).
For some reason carbon dioxide is relatively expensive to order by itself. I'd just suggest getting a whole comprehensive metabolic panel instead (they include CO2 and are around the same price).
Here are the companies I've found that allow you to order carbon dioxide directly:
Take a look at my state-specific issues page if you're in AZ, MD, NJ, NY or RI.