alt Hacker Newsand you can’t even find anyone who will fit them for the older models.
I'm quite certain you can find many companies in the far East who will produce cells of exactly the size and shape you want, as long as you're willing to order a minimum quantity. There are also a few semi-standard sizes of prismatic cells available.
That said, having a few truly standard sizes like we had with 1.2/1.5V and 9V batteries would be a good idea. BL-5C and its variants were a de-facto standard for many years too, and apparently are still available new.
I tried to find a phone battery once and found very similar looking ones with prices ranging from expensive to terrifying with everything in between. I don't trust the ones that are to cheap as I don't know how they cut the corners. I don't trust the more expensive ones because they look the same. I cant see the profit margins. I was unable to pick one. I ask a guy with a repair store. He said he always buys from the same shop and the badges look different every time.