The cells for the APC RBC7 and RBC55 UPS battery packs are all the same: 12V varying from 17Ah to 22Ah, has M5 leads for mounting the cables, and dimensions of ~76.5 * 181.5 * 167.5 mm
These cells are collectively named UB12180 but searching that term will get you prices that are not far from the APC ones, and APC wants to charge you an arm and a leg for them (around EUR 75 per cell).
Searching for the individual cell capacity will get you better offers.
Related blog posts:
Some links for finding them; maybe around Black Friday (it’s in a few days: 2024-11-29) I can really good deals:
The Wiert Corner - irregular stream of stuff 