Walmart has First Alert SCO2 Combination Smoke & Carbon Monoxide Alarm w/ 9V Battery (1039935) for $12. Shipping is free w/ Walmart+ (free trial available) or on orders $35+, otherwise select locations have stock for store pickup.

About this item:

• Electrochemical Carbon Monoxide Sensor – the most accurate sensor available as compared to other sensing technologies
• Two Latching features: Alarm latch to easily identify initiating alarm after alarm condition has subsided. Low battery latch: to visually identify which unit is in low battery condition.
• Two Silence Features: Alarm Silence to temporarily silence nuisance alarms. Low Battery Silence to silence low battery chirp for up to 8 hours.
• Intelligent Sensing Technology helps reduce the number of nuisance alarms
• Includes mounting anchors and screws.
• End of Life Signal provides audible notification alarms needs to be replaced
• 10-Year Limited Warranty