I could buy a bunch of those stops and mail them to someone in the US from where they could be sent onwards to individuals.

I actually found them in a rubber shop, not a hardware shop. I went to the rubber shop with a vague idea of finding some round rubber rods which I could then whip into shape on the lathe, but found the (home) doorstop instead. They also had a range of red rubber bungs the sort which is used to plug up lab flasks, which might have worked.

I have another idea up my sleeve which I still have to try out. Basically it is to use a 20mm diameter round rubber rod inside the original spring. As the spring compresses, so does the rubber, expanding in diameter and pressing against the spring coils. I am wondering if this might have a good enough damping effect.