Information was from GameShark Gabe, at http://www.railway.org/gt/gtcars.html, but extensively modified in format.

These are useful charts for keeping track of your progress. "N", "U", and "P" stand for "New", "Used" and "Prize" respectively, and indicate where you can or must find the car. I might get it entirely filled in some time. The important ones are ones where "N" is not available, but you must either find the car used, or win it as a prize. All such prizes seem to be deterministic (in fact, I think the Tuscan may be the only one you can only get by winning). With the used cars however, you may find no such car in the lot when you look.

Note that an additional aggravation is that very often prizes which it seems should be eligible are not. For example, the Mine's R34 Skyline GT-R cannot enter the R34 Race (although it can enter the GT-R Race!?!?!). (And it really cleans up, I might add )