Porsche sells replacement seat covers, but I'm sure they cost an arm and a leg.
Saw these aftermarket covers on ebay for $700.
http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&item=280196866549&ssPageName=ADME:B:WNA:MOTORS:1123 Never seen them in person.
There is a Boxster spec racing class that recently started. They take 1997-1999 cars and strip them, and remove the seats and the interior. That might be a source of donor seats.