The only ways I can think of are the rivals events (which might take a while for them to come round offering that car), photo competitions on the official Forza website, game with devs (I've never tried this so I'm not sure on exactly what you have to do). I'm sure there was another way but can't remember what it was or if it was just my imagination.
The only way I think you could get driving one is to find a club with one shared in the club garage but you won't be able to tune it, paint it or anything.
There's no real way to get the car instantly in your garage which is unfortunate, I'm still chasing after some of the unicorns I wanted from the start.