Amazon has the Kidde Nighthawk Carbon Monoxide Detector with AC-Plug-In with Battery Backup, Digital Display, model no. ‎KN-COPP-3, for $17.97 (regularly $49.99) with Free Shipping for a limited time.

This CO detector that alerts with a warning signal against dangerous levels of carbon monoxide in your home. Easy & quick installation – plugs directly into your home’s standard wall outlet and a 9V battery backup, the CO detector provides continuous protection from carbon monoxide, even during power outages. A red dot will appear in the lower, right corner of the CO alarm to indicate that the CO detector is operating normally. When the CO alarm detects carbon monoxide gas, 4 beeps will sound & a number reading will appear on the digital display that measures the CO concentration. These CO levels will be stored it the detectors memory until its reset. The digital display also provides a low battery warning by sounding 1 alarm per minute and displaying “Lb” on the screen. For whole home family protection, place 1 carbon monoxide detector on each level of your home & in your bedroom.