These things are called "Ölabstreifring" and, yes, you are right in that Audi does not sell them separately. AFAIK, they can be purchased aftermarket, for details try to ask Ronny Enzmann [
audiv8_32v@gmx.de ] who has changed his rings himself.
The cheapest solution would be to puchase and install a good used engine; for this, look in the
"Marktplatz" right here on this page. Good engines are < € 500.
