Used car purchase from non-Audi dealer
 

I'm looking for a 2013 S4. Lots of nice cars out there, but most are getting pretty close to the end of the factory warranty. I know I can buy a CPO car from a Audi dealer, which I would prefer to do, but if I find a car that fits our requirements from a private seller or a dealer other than Audi, what are the options for extending the warranty?