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.