SoCal

My personal suggestion is for you rethink your ideas on an unforgettable vacation. There are a few places that rent classic older cars, but they tend to be very, very expensive and require a roundtrip rental. Some major car rental companies, such as Hertz rent vehicles such as Mustangs and Camaros, but they are NOT the same as the classic cars of the past. And they won't always guarantee a particular model car. A convertible would cost more, be more subject to damage (and theft) and may not be used all that much in the desert heat. If you want a great road trip, I'd focus on what you'd actually see, rather than the name of the highway that some (not all) parts of your route used to be. You give no real info about your interests. If you're looking for beautiful scenery, look into the routes I, and others, have suggested. There are many other options, as well (e.g., Yellowstone, Rocky Mountains N.P.). But I get the impression you are bound and determined to find parts of Route 66, despite the weather, tradeoffs on scenery, etc. Perhaps you could find some of the mentioned fragments in Illinois, etc., suggested by other posters, and then take a truly scenic route to L.A. If you really want some more of 66 once you get there, I know there are some signs saying Historic Route 66 in some places east of L.A. (e.g., Ontario, Rancho Cucamonga). Remember, at the time you are traveling, the desert will be hot. Good luck and happy trails.