There are multiple tiers of batteries, I got the $50 one I linked but the first hit was $120 on their site.

So you can find the model’s battery here, and then google that and batteries plus. I have an 850M model which uses the rbc17 model. I’ve highlighted that on the apc site on this photo:

Batteries plus had a Duracell version of the one my APC unit needed, in stock in store so I could get it same day. If you’re in the US, might try them.