I actually came back to this thread intending to see/ask if anyone's managed to track down an OEM supplier of the horns.
I see the original ones are made by FIAMM, pic of my M3's horn
one and
two, and
No2 again showing the BMW part no.
Hmm, looking at the numbers on the top, the 2 horns are the same, I thought they were different tones for some reason.... [edit] yes they are 2 different tones according to
realoem.