Amazon has Kidde Tamper Resistant Plug-In Carbon Monoxide Detector Alarm with Battery Backup | Model KNCOB-DP2 for $14.07. Shipping is free for Prime members,
https://www.amazon.com/Kidde-KNCO…pons&psc
For comparison, $19.47 at Walmart.
https://www.walmart.com/ip/Kidde-…2/39360952