(1) As soon as Stevie wants you to get the Infernous go and get it

(2) However, don't take it to him! It's your only chance to do this....

(3) Drive around until other people have it or you see it on the streets and then take one that you find on the street or steal from someone.

(4) Then simply save the Infernus at your house and do the same thing to get more using the car you just saved...