Thank-you all for your assistance. I have found that the local dealer can supply the upper spring mounts for about $60 Canadian each. Before the screen flashed away, I saw that their cost is $36.

A local bodyshop guessed at about $500 or $600 for the whole repair, and has done it before on an older Mercedes. On that car, he reinforced the existing mount rather than putting on a new one. That is probably fine in my case, as the damage seems to be confined to the cracks, rather than generalized rust. He is going to have a look at it tomorrow.

Unfortunately I do not have any welding equipment, and not much welding skill, or I would try it, as Benzfan did. Maybe this is an excuse to get a MIG welder. I am sure it is OK to learn how to use it, on a critical suspension part.