household (potable) water pump spares

You could try and find places that supply and fit this type of pump and see what they do about spares support. My guess would be that they are assembled in the far east and imported by the container load. One or two bigger importers will possible stock a few spares but they will move so slowly, the accountants will insist on high prices such you have found.

Depending on your scruple levels, you could buy an identical new one, swap the defective parts and then exchange/get money back under warranty. However I could not possibly condone such behaviour.