Make one.
put something on it that you know the size of - 8.5"x11" paper for example - then take the pic
an 8.5 x 11 sheet of paper might not be the best way to go, if your scale is off by even 1/8" then you overall scale on an image that large will be off a lot. measure something on the vehicle that is easy to scale from like the front door seam to the back edge of the van. That way if you off by a little your scale will only be off a little not a lot.