Dollar rent convertible Mustangs, while Alamo rent Convertible Sebrings. Both are very nice cars. A Mustang is not so uniqure over in the States so most places probably do them. Check their websites.
Hertz in LAX definitely have Mustangs, as they were trying to get us to "upgrade". They have quite a selection of "top end" cars.
We recently picked up a car in LAX and dropped it in Vegas - had it for 2.5weeks. It cost €550 including everything. Good value, in my opinion. Think it was an extra €150 for the mustang. If you have an MBNA credit card, you get a discount from Hertz.