Some of us have been looking for replacements/substitutes for this little rubber thingie off and on for a while.
Suecanada had a possible solution, I think, but to my knowledge we haven't found an off the shelf, easily
found replacement. I suppose that if you contact the people at Palo Alto Speedo
http://www.paspeedo.com/bmw.htmthey *might* sell you just a replacement rubber bit, but I wouldn't keep my hopes up. They do a good job of
repairing speedometers, though.