Of course, everybody would say California. Due to the climate there, most older, used cars have little rust. Also, there is probably where most Porsches were/are sold new. As the demand for these cars has somewhat skyrocketed, however, you can find good examples all over the place. They travel quite a bit. You may find a good deal in the Midwest and South (Ohio to Kansas/Nebraska, Illinois to Florida) Most metropolitan areas are a good place to look.