I have a $60 SCD-30 CO2 sensor board[0] hooked up to a Raspberry Pi Zero W that serves up air quality data in chart form via Grafana. It’s not as portable as the device in TFA, but is a lot cheaper (at least if you already have a RPi) and you can easily look back at how the levels vary over weeks, months, etc.
By far the biggest source of CO2 in my house is the gas oven — its days are numbered!