Usually they're just 2 or 4 standard 12v 8a batteries out together and you can buy those yourself and take apart the previous battery back and redo the new ones from the taken apart ones. Might not be as pretty but it'll be cheaper than the official premade battery pack.
Discussion
What is a 'standard 12v 8a battery'?
Assume I am retarded
I say standard because they are used everywhere in panels for fire alarms, security systems, things like that for battery backup. Here's a link to one at home Depot. You can search for 12v 8a SLA battery. I'm sure there's reddit posts and YouTube videos of people diy'ing there UPS batteries
Thank you!