If you're in a large city, then no doubt there is at least one car rental agency that specializes in renting luxury cars. Below is the contact information for a couple of such agencies that we've had positive experiences with.
Houston Fantasy Cars
1202 Capitol St
Houston, TX 77002
(713) 199-1643
Black Diamond Exotics
1500 Rosecrans Ave
Los Angeles, CA 90266
(800) 965-6332