The only manufacturer I know of that offers parts like axles is QSMotors. Bafang can sometimes be bothered to sell parts like gears and clutches for their motors...but not always. But in both cases, they modify their products over time to either fix problems or decrease manufacturing costs, which means parts they make now may not fit older products.
However, the shipping costs for these, coming from China, can be pretty high.
FWIW, those economics (of scale, particularly) are what can make the parts as expensive as the entire motor or kit, for the end-seller to keep around and sell. Since they don't know how many they will sell over the years, and there are so many different variations over time of all the many parts of all the many devices they sell, they'd need an entire warehouse just to store all the individual parts, and hire personnel just to run that.
I'd love it if there were a "checker auto" or "napa" of ebike parts, but I can't imagine it ever happening--there's just not enough demand for the parts for it to be economical. Few people fix their own cars, but there are so many many cars using the same parts that overall it's economical for some companies to do this. The same isn't true of ebikes, whether OEM-built or DIY.
Unless you can convince lawmakers to require it to happen, and they can force a company to do this at a loss without putting them out of business (or it's subsidized by the government), or it is entirely government-run (so you pay for it in your taxes), I don't see any company choosing to do this.
There aren't even any ebike-specific junkyards I'm aware of, unlike cars, trucks, motorcycles, etc. If junkyards aren't profitable enough to have done, new spare parts warehouses almost certainly wouldn't be.
Maybe you could start a company that procures new or used or custom makes repair parts for people in your situation? Given the situation as it is now, you might be able to turn a profit doing that over a long enough time.
(I have a lot of gripes about the various ways the world works that it shouldn't, and ideas on how it could be better setup...but all of them require that someone be able to make more money doing it the better way than the way they do it now, and that isn't how any of them would work).