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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.
...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.
Thanks for the offer. I haven't given up on finding one here yet, and with the picture I have great hopes that someone can find a similar item. I'm pretty sure I can find tapered black rubber stoppers/bungs but they would need to be drilled out and wouldn't have the nice recess like the doorstop.