Are you talking about the Propane detector mounted low in the kitchen area? It is a wired 12V powered unit and they are only good for 5 or so years. Mine went out 4 years ago on my 2012.
The carbon monoxide detector in the bedroom is battery powered as I recall. They also have an end of life date after which they beep continuously.
In either case, here is a link to Amazon where they have a selection of propane and carbon monoxide replacements. I am sure you can find them elsewhere too.