If you want to get it from the GT world championship, do the save and reload trick, which is as follows...
Save and reload trick is this-
1. Finish all the races in a series but the last one. You'll need an 11 point lead on the next car down. Basically, win all but the last race. Then save.
2. Start the last race. Then quit it (be sure to start it first...the whole countdown, start, all that jazz). You'll get 0 points, someone else gets 10 points (why you need an 11 pt lead).
3. The whole spinny prize car thing happens. Try pressing X when the shadow's under the car you want. It works about 25% of the time If you get the car you want, go save and be happy
4. If it wasn't the prize car you wanted, don't save, go and reload your game from the main screen. This takes you back to after you won the 2nd to last race. Go back to step 2.
If you don't want to hurt your win percentage and get the completion percentage, you can just do the 10th race over and over.
If you aren't up by 11 points, qualify in 1st place and the next person down will only get 6 points instead of 10.