I'm fond of the Sensirion SCD30. Its low cost, low power, and easy to interface with. A lot of NDIR CO2 sensors pull 5V, so the sensirion's 3.3v power is nice. Digikey is currently out of them, but SparkFun has them in stock. Nice for hobbyists!
CO2meter.com is a good source for buying OEM CO2 sensors, although a little too coy with manufacturer's data sheets, in my opinion.
Virtually all NDIR CO2 sensors use some sort of automatic calibration, where the lowest reading in a certain period of time is assigned the value 450ppm. If that's accurate for your area and you can guarantee that the space the sensor is monitoring is unoccupied during that period of time, its not bad. Otherwise, a reference gas is the only way to really trust the readings.
CO2meter.com is a good source for buying OEM CO2 sensors, although a little too coy with manufacturer's data sheets, in my opinion.
Virtually all NDIR CO2 sensors use some sort of automatic calibration, where the lowest reading in a certain period of time is assigned the value 450ppm. If that's accurate for your area and you can guarantee that the space the sensor is monitoring is unoccupied during that period of time, its not bad. Otherwise, a reference gas is the only way to really trust the readings.