Stevie is an non-playable character in Grand Theft Auto IV. Whilst not encountered in person, he contacts Niko Bellic via text message to initiate stolen car delivery requests. He has a garage located in Bohan where the player delivers stolen cars in exchange for cash. Only requested vehicles are accepted until all thirty car deliveries are complete; at that point, any cars may be sold for cash. The reward is dependent on the condition of the car.
Some of the cars can be difficult to find, when I had trouble I found this website online called gta4onlineguide.com Then I had no more trouble! haha. It has pictures of the cars at their location as well as a zoomed in and zoomed out picture of the pause menu map pinpointing the car's exact location. It even shows you where the garage is that you need to take the cars for delivery. Check it out: