Walmart has Fire Alarms: First Alert Hardwired Smoke Detector $6.47, Kidde Battery-Operated Photoelectric Smoke Alarm $6.50 & More + free pick-up on sale for $6. Thank you Mike for posting.
Notes from Walmart:
Features: -120-volt hardwired smoke alarm wires directly into your home’s electrical system. -Ionization smoke sensor reliably detects particles from fast-flaming fires. -Gasket-less mount system for hassle-free installation. -Easy-to-access battery drawer with 9-volt battery included. -Can integrate with up to 12 other First Alert or BRK hardwired smoke detectors. -9V battery for backup if power goes out. Product Type: -Smoke Detector. Orientation: -Wall/Ceiling Mounted. Power Source(s): -9V Battery. Mounting: -Universal Mounting Bracket. Compliance, Standards: -UL Listed. Dimensions: Overall Height – Top to Bottom: -5.6″. Overall Width – Side to Side: -5.6″. Overall Depth – Front to Back: -1.9″. Overall Product Weight: -0.68 lbs. Specifications: Commercial OR Residential Certifications: -Yes.