ebay germany: ebay.de (search under audi speigel)
Plan on spending $200+ for a used set of OEM aluminum caps. Plan on wiring the $$ to seller if he/she doesn't take paypal.
Local (will be expenisive):
http://www.epimporters.com/products/...tion.php?II=57
FYI - The B5 S4, C5 S6 and D2 S8 all share the same mirror type (forged polished aluminum).
Search also the C5 A6 forum on this topic for install help (pretty easy, I've done them 3 times) and part numbers etc.
Good luck!
PS: If you want the cheapo plastic versions, check ebay.com. You'll find a bunch of sellers.