We had several of these mushrooms completely missing when we bought our SM ~2 years ago and could not find replacements anywhere. My extremely clever wife Denise came up with a "temporary" solution that is still working to this day. She used Sugru Mouldable Glue to make her own mushrooms and they have been working great for 2 years now.
In the USA it's available online at Amazon, Target, Walmart, eBay, etc. However, its a UK based company so I am sure it available in the EU/UK as well.
Some pics of one of her handmade mushrooms as well as the Sugru box are attached below:
We have also have had surprisingly good success using this to fix some broken "fingers" on cabin top lockers. It really is great stuff to have around the boat!
SM #440 Cara
Deale, MD USA