I've never rented an exotic, but I have seen them on the web: http://www.exoticcarrentaldirectory.com/directory.htm http://www.drivenimage.com/index.asp ...of course, you could call the press office at a manufacturer and tell them that you're a writer for some magazine looking to do a "road test".
Drivenimage rents them, I know that they have a brand new Red 360 spider with an F1. I would say arrange it to be in better weather though. Good Luck