The Verizon website states that the 12 Volt 7.5 Amp Hour lead acid battery used in the FiOS Battery Backup Unit (BBU) is available in Home Depot, Lowes and Radio Shack.
A friend tried to purchase one and was unsuccessful. None of the three companies lists the required battery on their website. I just visited a local Lowes just north of NYC in a FiOS area. They do not stock them.
I know that some burglar alarm & security system companies probably carry them. I've found them on Amazon and eBay.
Since most FiOS installations are no more than a year or two old, there's going to be hell to pay and lots of very unhappy subscribers when those batteries start failing and customers try to find batteries where Verizon says they are. They aren't.