https://www.ebay.co.uk/itm/394467949152?mkcid=16&mkevt=1&mkrid=711-127632-2357-0&ssspo=jmlf3a-qstq&sssrc=2349624&ssuid=gzuTA6oyTqi&var=&widget_ver=artemis&media=COPY

This is UK, but find something similar in the US and you'll be fine. Frankly it's worth having in your plumbing supplies box.

There are other ways of estimating* atmospheric co2 for dates prior to we can access using ice cores. Off the top of my head some include alkenones, boron isotope ratios in microfossils, stomatal counts in fossil leaves. They do have significantly greater uncertainty than direct ice core measurements of course.