PiotrovThere is a TDU section but to answer your question, the car will be at the used car lot. If the lot is full you have to buy one of the other bars and then it will be there.
Just do what i did here http://www.youtube.com/watch?v=PhpaReofYHMI bought the Shelby GT500 and downloaded it, how do I get it? I heard you still have to buy it from the dealerships, but it's not at any of them or at any of my houses.