Heres a nice site, they dont have a murcielago but several Gallardos:

Luxury car rental, Exotic and Luxury Car Rental, Exotic car rental - Los Angeles, Phoenix & S. CA - Las Vegas - New York

Murcie Coupe:

Lamborghini Murcielago Coupe for rent in miami palm beach los angeles

Coupe and Roadster:

Rent a Lamborghini Murcielago Gallardo More


Just a Gallardo but pretty cheap compared to the others:

Rent a Lamborghini in Los Angeles - Exotic Rental Cars


Hope this is helpful