Ues the search function! It's not there to look pretty!
If you want the R8, you need to win one of the Le Mans 24 Hour events. To do that you'll need to spend 4,500,000 credits on an LMP racer (which is how much the R8 costs) to stand a chance, or win one through one of the other enduros, like the El Capitan event, which you'll need a decnetly-powered car that can kep up with the likes of TVRs and Mercedes.
Alternatively, win the DTM event to get the Mercedes CLK-GTR. Not as powerful as the other LMPs, but still a damn fine machine in its own right.
The point is, it's not going to be easy ... but then, it's not meant to be. These are the ultimate machines in GT4, so you'e not going to win one simply by cruising through a one-make race.