Therein, as they say, lies the rub.
AoA doesn't stock them anymore (call around, you may find a stealership with one sitting on the shelf), and OE suppliers I've found are selling an Interchange part that doesn't interchange properly. If you can stomach having your RS6 down for a week or so, you could have your Alternator pulled and send it off for rebuild - I've used Kaestner Auto in Wisconsin, and they've been awesome thus far. They're the ones who found the issue with the non-interchangeable part - the pulley shaft is too small of a diameter and doesn't stick far enough out of the alternator to have the proper pulley mounted. As for location/changing - the alternator is under the engine and is an enormous pain in the ass to get to / R&R. I took mine to a local shop, they charged 6 hours labor and that was on the low side since they know me and have worked on my cars before.
Good luck!
O_E