There are a bunch of raw sensor components that you can connect to an Arduino or Pi. I ended up going with the Sensirion SCD30 but also tested options from Amph

There are a bunch of raw sensor components that you can connect to an Arduino or Pi. I ended up going with the Sensirion SCD30 but also tested options from Amphenol's Telaire line. Both are pretty easy to communicate with using an Arduino. I haven't tested an MH-Z19 but that's a popular cheap option.