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

Manufacturers' Races
West City
ManufacturerRaceNUPDoneComments
Alfa Romeo 155 & 156 - Normal N    
155 & 156 - Racing N    
Citroen Saxo Challenge - Normal N    
Saxo Challenge - Racing N    
Fiat 500 Meeting - Normal N    
500 Meeting - Racing N    
Lancia Delta Cup - Normal N    
Delta Cup - Racing N    
Peugeot 106 Challenge - Normal N    
106 Challenge - Racing N    
Renault Clio Cup - Normal N    
Clio Cup - Racing N    
Venturi None  

"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.

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 )