I'd never let someone test drive my Z. Seeing vids of people absolutely trash rental cars on youtube is more than enough. Plus, the one time I ever let someone drive one of my cars, a camry, she hit a lightpole while parking....with me in the passenger seat
I agree with the others, definitely try dealerships. When I was buying the Z, several dealerships were fine with me test driving the Z for the entire afternoon. I'm sure you can find one that would be willing to let you test drive the whole day if you showed that you are serious about buying.