Hi there
Ricambio International who now operate an ebay store specialise in old 500 and 126 parts but also 600 and 850. Do google search and you'll find their details, is run by Franco and Felice, they used to have a shop in Banstead, just online now I think.
Theres also a German company called Axel Gerstle or something, he can supply absolutely anything.
If you can't find contacts googling let me know and I'll dig out ther telephone numbers.