Try searching on http://www.brandsoftheworld.com
You can download vector graphics from that website. I have personally never downloaded anything from them, so I cannot vouch that the files will be virus free. Also copyright laws vary in different countries, so reproducing a logo may be in violation of those laws.
If you are trying to mimic a current race car, you might contact the race team directly and tell them what you are wanting to do. Chances are they might be flattered by your efforts and provide you with logos/decals that you can use.
Good luck with your efforts.