Every shop stock them, you can buy them from Madison.
Sadly I'm not from the UK! Local shops didn't seem to have them and having such a tiny part posted didn't seem worth the effort, so I just made my own. Works perfectly.
In that case, local shops should be able to order it from one of their supplier (our is usually Madison), obviously you got a solution, but if another one is needed, LBS really should have some.
Every shop stock them, you can buy them from Madison.