Haven't found a template that was accurate yet... unless I made it. For most vehicles, we use digital pics. Just snap a pic of the side of the vehicle with the camera centered horizontally and vertically -- meaning you'll have to hold the camera about waist high in most situations. Take one reference measurement and then scale that photo up to actual size in Photoshop. We actually scale it up to 1/10th size at 1000ppi. PS handles the files a little better and you wind up with a print ready file that is 100ppi if you scale it up 1000% in your RIP.
For cars with crazy curves (like a Bug) or boats, jet skis, etc we make our own paper templates then slap them up on a wall and take a digital pic. Remember to draw something on your template that you can use to scale with. We typically mark off 12 inches. Take it into PS and scale your mark up to 12" (or 1.2" at 1000ppi) and you're good to go.