I think most of the big rental outfits rent them. We went through Hertz (I think) rented a cheap on gas Smart 4-4 as we did nearly 2000km. but they had 8-10 Porsches availiable for rent at the Airport in Frankfurt. I didn't ask the price. I would gladly payfor the ring taxi experience with Sabine. Just couldn't work it in at the time.
http://www.youtube.com/watch?v=jQFNQtOxPto&feature=related
http://www.youtube.com/watch?v=m1pklvKKnd0&feature=related