Being that type of guy, a red LED glowing in the far corner of the basement attracts my attention:
Back in the day, Verizon didn’t make it obvious that the customer is responsible for replacing the battery keeping the ONT alive during power failures. I expect VZ would eventually let me know the battery was dead, remind me I was on the hook for the replacement, then offer to send a tech around with a Genuine VZ Battery to maintain reliable service.
It’s an ordinary 12 V 8 A·hr sealed lead acid battery and, yes, it’s been in there for a while:
However, being that type of guy, I just happen to have a box of not-dead-yet SLA batteries waiting for recycling:
They’re three years younger than the VZ battery …
The Smell of Molten Projects in the Morning 
When I needed a replacement battery for a dead UPS I learned that SLA batteries can be cheaper if you don’t look for a UPS battery but go to a store that has a fishing section.
Other sources for new batteries: 1) The Batteries+ people offer(ed?) them online. When my ancient Belkin UPS needed new ones, I went that route the last time. (OTOH, it’s sitting in the shop, twiddling its thumbs. I’m relying on the Linux boxen to behave in a power outage. It’s been working so far, and the house is Windows-less. OTOH, I should look into shutdown scripts for a power fail.
2) Electric fences use(d) such batteries, so if you have a farm & ranch store in the area, you might find them. I did that for earlier replacements, though the last time, they only had one of the two I needed.