*SLIGHT SPOILER ALERT!*
I have just recently finished the story part of GTA4 (PS3 version if it makes any difference). Now i am just doing the side missions, including getting Stevie the cars he wants.
Ok, getting straight to the point, I have read many comments and watched many videos to get Infernus, but I feel that they are not clear enough.
There are a couple of ways to get the Infernus:
1. You are given one after completing all the missions for Bernie/Florian.
2. Call Brucie to hook you up to a race and take one from there, but from experience this is rather hard, because the type of cars racing is always random and it takes about 20mins before you can get another race.
3. Taking the Infernus that Stevie wants you to look for, once you've taken it and placed at your safehouse, the same one should respawn. However, once you've taken the Infernus and given to Stevie the car is no longer there. I have an image of it available here marked with the red dot, it should be outside a "PIZZA SALAD" restaurant.
4. Bohan cloning; this is my favourite way, but requires an infernus.
Here I am going to elaborate on method 3...
First,you must have an Infernus to begin with, use either method 1, 2, 3 to get the car.
Second, drive the Infernus to Grand Boulevarde towards the Northwood Heights Bridge.
Third, make sure you make a scene around there. Just do a couple drive-by and bump into a cop car to get 1-2 stars (enough so that the game acklowledges you and the car you are driving [infernus]). Normally I just cause a traffic jam and do a drive-by at those cars and throw a bomb at them, then the cops start to get me. Then i usually drive along the Northwood Heights Bridge making sure the cops are still on me and shooting at me, the Grand Boulevard is out of sight. After having a little bit of fun with the cops and using the cheat to get rid of them, then I drive back towards Grand Boulevard and there is usually a line of them parked on both sides and sometimes on San Quentin Ave. Here is another image of where i find the infernus clones.
Fourthly, when you have found the infernus clones you must hi-jack it and drive them for short drive or just push one in front of the other. Do not let either one out of sight or it will disappear.
I would recommend that you keep at least 2 infernus, one to keep, one to play with. I initially kept my first two infernus at the Bohan safehouse, so i can use one of the infernus to get more, then push one of them back to that safehouse and drive the other to another safehouse. I repeated this several times and have got 7 infernus (with and without spoilers on the back).
I hope this help people trying to get infernus cars. They are a treat to drive but tad bit annoying to obtain.