Hi Giorgio,
Up till now I have found everything I wanted and/or needed on these two addresses in Holland;
De Groot is in the North of Holland about 1 hour drive from Amsterdam. They don't have a shop, but they are very friendly people, so if you like to pick up some parts, just give them a ring. They have virtually everything in stock
Peter's classic bike parts is in the near of Rotterdam. His shop is open Thursday's and Friday's from 10.00 am and it's a real Aladdin's cave
Just check out these websites for more information;
http://www.degroot-bsa.nlor;
http://www.petersclassicbikeparts.nlThe address and phone number can be found on the website.
Good luck on your trip!
Patrick