The plastic ring illuminates the lighter when the headlights are on, and the ring in my '62 Giulietta Spider is identical to that in my modern Mercedes. I've been told the same is the case for modern BMWs. So you might try an MB or BMW dealer, but you would probably have to buy the entire lighter assembly.
if you don't mind replacing the entire unit, a lot of auto stores sell a complete unit, same style w/illuminating ring and lamp holder, bulb & wire for less than $10. I've seen it available w/amber ring and also with a very nice soft green ring...It's not a "metric" hole size but is slightly smaller so it fits the assembly w/o any modifications