The replacement style might vary based on whether you have a combination battery backup/power supply or two separate units. The separate units take a 7.5 ampere-hour 12 volt sealed battery (7.25 AH or greater is fine).
Or you can get a cheaper generic replacement here: »www.batterymart.com/p-12v-7ah-se···-f2.html
Or here: »www.batteriesplus.com/pc-32634-7···.5F.aspx
Or even look on eBay ...
Or you might even be able to buy one from FairPoint - but good luck with that.