[Edited by TheKahn, 8/16/2008 9:09:37 AM]
You can't , you have to defeat Wolf[again] when he's in the north team[forgot its name].
Stacked Deck i think.
If you mean the Lamborghinoi Murcielago LP640 custom car, then you need a trainer that unlocks all cars for purchase and buy it that way.
Note it's not a dominant (not the fastest) car in the game, but just about any tier 3 car is good enough to finish career, with the possible exception of some of the slower muscle cars.
For online play, most players use the Corvette Z06 or the Audi Lemans. The Z06 is a pain to drive though.