Batteryhookup and Jag35 are hobbyist-friendly places dealing in such things. Safety or documentation are very much YOYOMF.

I can vouch for battery hookup, they even sell new/never used overstock sometimes. Pretty good way to build your own battery back up system, which has helped me a lot when we lost power here in quebec for 4 days last week.

Probably not at scale due to all the different form factors. There are plenty of hobbyists doing that, however.

You can buy old Tesla modules (typically from wrecked cars) on eBay. Once in a while I see people on the RV forums use them instead of LFP, though honestly I'd much rather have LFP for RV or home energy storage than regular lithium ion.