There was a thread on this a year or two ago, and a lot of research was done. I don't remember all the details at this distance, but the conclusion was that "domestic" CO detectors are not worth it in an aircraft - by the time they "trigger" and you notice,it's too late.
There was a model recommended by one of the US organisations (Aeromedix IIRC), which I arranged to buy in bulk for a bunch of us. That turned out to be
too sensitive: it would go off if someone taxied past and the wind was blowing towards it. As far as I know, none of those units lasted the "lifetime" of two years. The one in our aircraft died after a few months.
There's a more recent recommendation for an aviation one, made in the UK I believe, and available for around £50. Memory says it's Senco but that might be the haze. Anyway, lots more info here:
http://www.avweb.com/news/aeromed/186016-1.html