How do you find some of the rarer cars?
Check the online shop. Seriously. Doesn't refresh often, but there's some oddballs for sale there.
Grab a bigger pool of cash. Rarer stuff will pop up, but you still need the level to actually buy it. I recommend playing the 2nd race in the professional group. (Name escapes me atm). German cars only, but you get like 40k per round (approx 6 mins) and the highest HP car in that race is only 200HP. (assuming you're around at least level 10-15)
Save up, buy the GTR '07, deck it out, and then there's a tuning race I think it was, for about 4 mins of driving you get approx 50k. Save up and begin buying stuff.
Damn, even at 700HP, the GTR is faster than a lot of cars with equivalent or higher HP.