These cost around £7 each and can be ordered from any BMW dealer: Part # 91.47.3.610.086
Photos and captions below courtesy of Ade Flint:
Here is a pic to show the difference (old valve on left):-

And a pic of the parts breakdown...
you can clearly see the PTFE washer in the new assembly (upper items) used as the new interface with the wheel compared to the old stainless one:-
