IMO Central Dispatch is the front runner, ie best known Car hauling site out there. The other freight sites dont have a whole lot in the way of Autos. Other sites that I saw were
www.carsarrive.com and
www.movingloads.com Im not sure if either are any good, or not but maybe it helps.
I would try the large auto auction groups like Manheim, etc and start plugging your name around. Anymore with the internet, a dealer in say Chicago isnt afriad to online-bid on a car in say New York, so it may be a way to go, start advertising around these places, and or trade magazines, etc.