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.