Stevie's Missions
Play Stevie's car-theft missions until he requests an Infernus. Once the car is acquired, do not take it to Stevie. Instead, take the car to a safe house and park it in an outdoor resident-only parking space. You can then go back for another Infernus, and infinitely more, as long as the mission isn't completed.
Street Racing
During Brucie's street races, the opposing cars are randomly spawned, meaning there is a chance that one of the other racers will have an Infernus. The spawned vehicles are also found on the streets during the race. If one of the racers has an Infernus, use the race time to steal one. You can quit the race or win it and keep the Infernus.
Telephone Cheat
An Infernus can be spawned using a cheat code on Niko's cell phone. However, it has only been confirmed on the PlayStation 3 and not on the Xbox 360 or PC as of 2011. Go to the Liberty City airport and press up to bring up the cell phone. Dial 989-559-0550 begin_of_the_skype_highlighting 989-559-0550 end_of_the_skype_highlighting. Enter the airport parking garage and go up to the first floor. The car is located in the lot.
Bernie's Reward
For a non-cheat method of obtaining an Infernus on both the Xbox 360 and PS3 complete Bernie's final mission, "Buoy's Ahead." As a reward, Bernie offers Niko Bryce's old car, an Infernus.