There certainly are, although they're not typically intended for home usage. But in industrial/commercial setups which use CO2 (beer/drink-cellars, dry-ice handling, pressure pipe-flushing, etc) they're relatively common. Obviously they're always detecting some level, but they're set to trip or alarm when concentration hits some threshold over ambient. http://duomo.co.uk/CO2_Alarms.aspx is just one example.
Low-O2 detectors are also common, and approach the problem from the other direction. They're more useful for N2 or other inert gases, since you're less concerned with toxicity and more with plain asphyxiation. IIRC they're used in some places to enforce "No riding in the same lift as the liquid-N2 dewar" safety rules.
Not really, you can buy CO2 sensors COTS for HVAC guys to prevent "stuffy" rooms. Its industrial grade product and price and you won't be buying them at walmart but the UI is very HVAC-dude oriented. Also if you sell dry ice you're supposed to own a permanently wall mounted one even if you officially keep the stuff outside (depends on local building code and licenses and regs I'm sure)
There's a different UI for combustion gas analyzers, most any HVAC guy owns one if he does furnace work. They're about an order of magnitude more expensive and the ratio of CO2/O2 tells you quite a bit about the level of draft (and they usually have a CO sensor too which only comes into play when draft/mixture is tremendously rich)
Low-O2 detectors are also common, and approach the problem from the other direction. They're more useful for N2 or other inert gases, since you're less concerned with toxicity and more with plain asphyxiation. IIRC they're used in some places to enforce "No riding in the same lift as the liquid-N2 dewar" safety rules.