Hertz do still have Mustangs in their Fun collection which guarantees the car type - I saw one on there the other day when I was making a booking.
However I'd recommend the Chevy Camaro (unless you specifically wanted a convertible) - much nicer to drive than the Mustang. There is a Camaro convertible coming out, but not sure if it's available as a rental yet.
Nationalcar also have Mustangs, and if you sign up with the Emerald Club, they let you choose your car from whatever is available. So whilst they don't guarantee a particular car, you go to the aisle that contains your booked type, and choose which car you want! There will usually be Mustangs there in LA anyway - they're a dime a dozen!