I have a 'roll-your-own' using an adafruit SCD-30 module https://www.adafruit.com/product/4867 IR-based CO2, temp & humidity; I2C with python libraries, so integrating it with an RPi is easy. Sensor is self-calibrating over time, so if you leave it in a higher CO2 space with no exposure to fresh air, it will eventually drift such that the lowest observed CO2 reports as 420 ppm. Newer SCD-40 is only $45, but different sensor technology.