Try asking your dealer in the first instance (always the best option) but, if your dealer can’t help, a telephone call or e-mail to Naim in Salisbury is probably most likely to hunt down the pesky little thing.
You might also try an independent service agent such as Class A in Sheffield, or even a retailer like TomTom in St Albans.
Fair enough. James at TomTom is a decent chap, so I would drop him an e-mail (if I were you), as I’m sure that he would try to help - after all, a plastic knob (if he had one in stock) in a jiffy bag wouldn’t be difficult. Up to you, though.
Modena, please note forum rules re. posting wants - strictly forbidden on here.
I would try Naim first then try ringing around some of the more likely dealers. You could also keep an eye on places like eBay and the like. This kind of thing sometimes crops up, but you do need to be patient.
If you can find somewhere that does 3d printing they can use the other knob as a pattern. Should be cheap and they should be able to reasonably match the colour. Worth a punt even as a stop gap.