Apart from the normal sources, Greg (OCT, 3CMoto, Marty in Dallas, etc - I guess you know 'em all by now), you could try your local :-* Marchesini dealer.
I haven't looked into this myself; but these, for example,
could be the same. If you do, do please let us know what you discover

.
http://motowheels.com/i-22057306-marchesini-5-spoke-cast-magnesium-replacement-cush-drive-rubbers-set-of-5.html
Ciao
Really good point about following through with Marchesini. The site you sent has sets of 5. Zanes have 6. Good pointer though. When I find them I'll post the contact.
Regards, Greg
Craig