I dont want to discourage you, but some important things you've got to remember.
-3d printing an object that large requires a massive printer, I'm not going to insult your inteligance, I know you already know that, but what you might not have realised is you cant just split it into small parts and stick them together. That'll make serious structual weaknesses that could result in an accident (see my next point)
-3d printing is not strong. I have rarely come across a 3d print I cant snap with my hands. Even at 100% infill its not that strong. Just consider what might happen if your spoiler broke while driving. If the surface isnt perfectly smooth and even it'll have a lot of twist force on it, on top of directional force of the air.
- Finally, this would cost a lot in materials, time and actual cash.
I dont want to squash your enthusiasm, but you need to really think about the safety before doing anything functional like that
Yes, am aware of that. The point is to basicaly create a surface to laminate carbon fibre onto. The carbon will basically be cosmetic as well as structural with the printed object giving the shape to laminate to and add stiffnes to the completed part carbon and ABS
There are many sites where it could possibly be found. I would suggest googling for a 3d model of your specific car and go from there. Be advised though, most highly accurate models you will have to pay for.
What kind of printer do you have...A Big Rep?
When it comes to the build-up, I would print it with about 30 to 50% infill and no top or bottom layers so the resin and hardener will fully penetrate the buck (the printed master). I have done a lot of custom automotive fiberglass work over the years, and without the fill-in between surfaces, you will end up with a very fragile part.
If you cannot find a 3D model of your car I would suggest digitising your car as the mount points etc are important. For a 'proof of concept' you could use photogrammetry as there are some free ones out there (all you need is a camera and the software).