Can't help you with a shop or cost.
The regulator is usually what goes out, and Porsche does not sell just the regulator.
http://www.renntech.org/forums/index.php?showtopic=3311
Porsche sells a new or rebuilt alternator, and both are expensive. It is just a Bosch 150 amp alternator used on other German cars. You might try local parts places for a rebuilt one. Here in the US parts stores such as Autozone sells a rebuilt Bosch for around $200.