I have an Airthings Wave. It does CO2 along with a couple of other things.

I really like mine. It helped me discover that I was sensitive to volatile organic compounds: every time I felt the air was stuffy inside my apartment, it corresponded to a VOC peak that was happening right then.

The one downside to the Airthings Wave is that it looks like hospital equipment. Grey, lifeless plastic pill design that was probably outdated even in the 1970s.

If you ignore how it looks, it works fantastic.