Comment by warp

I'm using a co2meter.com one [1] connected with USB. Reading out the values is a bit slow, but I just do that in a cron job every 5 minutes and log the results. This does exactly what I needed it to do with a minimum of fuss, so I'm happy with it.

[1] "CO2Meter RAD-0301 Mini CO2 Monitor" on Amazon, $70