If possible I try to shoot the vehicle myself or start from scratch. The problem with starting with a photo is the perspective is too wide and the sketch looks like its all jacked up, and looks like it was over a photo. I like to sketch in views that have a bit less perspective, that's just me though.
Mark
There's a slew of good car pics at Flickr.com too. I post a lot of mine there. You can save really big images and others can download the really big (original) sizes too.