The one I use is made by BW and is called GasAlert.
The best URL I can find right now is to a version of it but it looks the same:
http://www.gasmonitors.com/main.cfm?...odpage1&pid=31
It costs about £300 and the battery lasts a whole year of continuously ON operation.
It's excellent, even picking up CO emissions from the plane that landed before me.