I am purchasing a '03 Audi S8 equipped with the Audi Navigation Plus system that was installed aftermarket. The owner does not have the disc for the system. Any suggestions where these could be purchased?

Here is the place that sells the discs. You can also purchase them from the Audi dealers.

http://www.navigation.com/is-bin/INTERSHOP.enfinity/WFS/Navteq-NavteqNorthAmerica-Site/en_US/-/USD/ViewStandardCatalog-Browse;pgid=kHFqUX2YWgVSR04GFKjzG8Ko0000spBbOXoU?C atalogCategoryID=SRAKCggRMbEAAAEO5DNtNDzr&ShowAllProducts=YES