Ring Alarm Smoke & CO Listener (1-Pack) White 4SS1S8-0EN0 - Best Buy
X-Sense Wi-Fi Listener for Smoke & Carbon Monoxide Detectors, Free Real-Time Alerts, Works with SBS50 Base Station (Included), Not for Smoke or CO Monitoring, Model SAL11 - Amazon.com
Ring Alarm Smoke and CO Listener in White | NFM
Ring Alarm Smoke/CO Listener 4SS1S8-0EN0 - The Home Depot