Mom's carbon monoxide detector started chirping, so she picked up a two-pack of batteries at Walmart. I would not expect to find cheaper prices elsewhere. I do not know if our local dollar store has them, but Dollar General and Family Dollar show $8 for two on their website, just like Walmart:
Duracell Coppertop Alkaline 9V Batteries 2 Pack $7.98
She noticed that they were $18 for two at Safeway.
Amazon to the rescue!
AmazonBasics 9 Volt Performance Alkaline Batteries - Pack of 8 for $10.99
Which would have been great if we wanted to listen to the chirping for 48 hours.
By the way, we completely forgot that we bought a carbon monoxide detector. We thought it was the smoke detector in the kitchen, by Mom's bedroom door. Then I tried the one just inside her door. I did not think that it was the one in the tiny hallway on the other side of the house or just inside each of our bedrooms, each five feet from the one in the hallway.
Why are they in clusters?!
So, at one point, I had all five smoke detector in front of me on a table, and then I heard a chirp from somewhere behind me.
Eventually Mom remembered the carbon monoxide detector [that I bought] and said that it was plugged into an outlet somewhere, but there were definitely not any detectors plugged anywhere near the chirping.
Then she found it broken on the counter under a loaf of bread.