You can buy it at an online shop like eBay or aliexpress about 20$.
The electrical interface of this sensor is compatible with Raspberry Pi. So you can make your D.I.Y. CO2 Monitor with MH-Z19, Raspberry Pi.
The normal CO2 concentration is about 400ppm. On a global scale, this value is sensitive to the effect of the activities of the plants, seasonally it gets lower in summer due to vigorous plants photosynthesis and getting higher in winter, and the long-range trend is continuing to rise linearly. The Japanese Meteorological Agency reports CO2 concentration chart continuously as hire. It indicates a clear trend of rising.
On your room-scale, in a closed room with 2 people, CO2 concentration rise up to 1,200ppm soon and keep falling slowly after 1 person go out.
In a high CO2 concentration room as 1200ppm, somebody can get a headache or sleepiness.
But no need to concern about…