Have a saved game file from Sega GT 2002 on the Xbox hard drive. Go to the "Load File" option under Sega GT mode, then select "Perk" to get extra money at the start of the game.
On a day that ends in a five (for example, 235), go to Event Race. Enter the race, then lose it. Go back to the screen with the "Official Race", "Event Race", "Garage", etc. options. It should read as the next day (for example, 236). Then, go to the used part store. Go all the way to the end after the turbo and there should be a "???" box. Select it and buy the Nitrous. Note: You must have a turbo to get it. If you have a turbo and it does not appear, exit the screen then go back in again. You can now go to a drag race, and while you are driving, press Y to activate it.