Exactly which Island are you on? Are you doing the Turismo race for the Diablo's on the First Island or the Race for the Yardies (I think!) on the Second Island? Its been so long since I played this game, for the life of me I cant remember it properly... Anyways if you are on the Diablo's Turismo 'Mission' just use the Sports Car inside the Showroom of the Car Dealer near the Fire Station and next to 8 Balls pad.. Again I would tell you the name of the Car, but I cant remember it all... Cheetah?? I dont know...
If you mean the race on the Second Island, then just use any of the Sports Cars driving around.. Personally I would go with the Infernus...