I'm a little surprised that no one has mentioned Aranet yet. From what I can tell, they appear to work quite well. However, it is quite pricey.

I did look into the AirGradient when geerlingguy put out a video about it but they were out of stock at the time. It looks like there might be a new version but that may be something worth looking at.

I tried the Vitalight CO2s but the readings were all over the place despite recalibration with 2 devices side by side. Ended up returning these. Do not recommend.