To get the nitrous from the used parts shop, you will have to lose at the drag races on a day ending with "5". For example, 305,15, etc. Return to the screen for the parts shop after the drag race. It will now be 306,16,etc. There will be a question mark at the end of all the parts, after the turbos. If your car has a turbo or if it does not appear, get out then go back in and it should appear.