Pro Vehicle Outlines is subscription based, you pay annually for the entire collection, great if it's your bread & butter, but not so cost effective if you don't use it daily. Bad Wraps can be purchased individually, which is good if you only need a template here and there. There are a lot of sites that sell templates, some subscription, some collections you buy, some sell individuals, some just do specialty vehicles, race cars, atv's... Check reviews to see if there are overwhelming complaints about quality or inaccuracies.
Most are pretty accurate, have specific layers to mask your designs to, but always measure the vehicle too, it's the only way to know it's accurate, and calculate how you want to set up your print files, where you'll want additional bleed, etc. Either style will give a good mock-up for a customer to show what it'll look like. They're at smaller scale, to create your print files you blow them up to size, add your bleed, tile sections to print size, the scale each uses is always included. Any design software can be used with these templates that uses vector layers, and that you can accurately blow up to full scale, like Illustrator, Corel, Affinity Designer...
If you've never done one before, it makes the designing part fairly straightforward, but takes a bit of playing and calculating to get print files set up right, add bleed & stuff where you need it to work, so be prepared to eat some time on the first ones, and probably a few reprints. My first couple were sort of a nightmare, but that's the learning process.
