We use Pro Vehicle Outlines and on rare occasions, Bad Wraps.
PVO - They're great, easy to use and mask in illustrator, and only rarely do I have an issue with measurements. If they don't have an outline of a vehicle online, they're super speedy in getting you an outline so great customer service with that!
We push our clients to give us photos of all sides of the vehicles as the usual "year, make, and model" question doesn't give you all the details. With photos or them coming to our shop for us to measure, take photos, and talk design, we can be sure that the layout matches the clients vehicle.
Bad Wraps - We use this if we absolutely cannot find the vehicle outline on PVO (even after contacting PVO in my experience). Cons for this is the use of photoshop and really longggg loading times (maybe it's just our computers). Overall, the file for these guys are too large for us to handle so we don't favor it though the layouts look really good and realistic.
I think they have a limited selection compared to PVO. I've seen that PVO has lately, included a lot more categories in their library..