CO2 sensor recommendations?

[Sparrowhawk]

I want to add CO2 concentration to the data I currently monitor (power, temperature and relative humidity). I've already discovered how inaccurate humidity sensors can be and the need for calibration, and would like to start with CO2 sensors that are likely to be accurate.

 

Zigbee support preferred as that's what everything else is using.

 

What have people experience with?