replacement CO detector for use with Elk?

[asteinmetz]

My Codet 12-24 CO detectors have reached end-of-service-life.  They are supervised low voltage gizmos that are wired into my Elk alarm (http://www.us.sbt.siemens.com/faraday/pdfs/9692.pdf).  I only see one online source for them and wonder if they have been superseded by a new model or there is a more economical alternative.
 
Can anyone recommend an drop-in alternative CO detector for use with Elk that uses the same form factor.  Thanks.

 

[Del91574]

Potter makes a CO1224, however for practicality and ease of locating, look at System Sensor's CO1224TR