I'm using an NDIR-based USB-CO2-meter (~40 €, allegedly USB only for power, but it actually spits out measurement data over USB) and I think it's pretty accurate. When I put it outside it generally reflects ambient CO2 pretty accurately (440-450 ppm) and I found through blind-testing that "air staleness" correlates very well with CO2 concentration. For me, air feels noticeably stale at around ~700-800 ppm, which seems largely inline with academic results.

You can actually see when the device acquires a reading, because the tube flashes.