In theory exact sources can't be posted or named, but since this is stuff that can't be used to CTB (just to
test the purity of a gas), I will say it:
▪ O2BOX from Forensics Detectors (I have this one)
▪ CY-12C (I have this one too, it's the one used in the PPeH)
▪ CY-7B (a most recent model of the CY-12C)
My CO₂ detector is the HT-2000