I’ve been looking for smart carbon dioxide sensors, particularly sensors compatible with Apple’s HomeKit, but they seem to be fairly scarce on the ground (at least compared to carbon monoxide sensors). So far what I’ve been able to find is bundled with a lot of other sensors. Netatmo’s Weather Station has an outdoor module and up to three indoor modules: the indoor module includes a CO2 sensor. The Airthings Wave Plus includes CO2 as one of a half-dozen things it detects (it’s primarily a radon detector: the rest are kind of thrown in). It’s only HomeKit-compatible via a Homebridge plugin, but that’s a viable option: I’ve got a Raspberry Pi I can install Homebridge on. Problem is, neither is particularly cheap, nor remotely as cheap as a standalone sensor would have been.