No vehicle outline library is 100% accurate. it cant truly be in the 2D format it comes in.
It should be used as a guide, but still very good at that.
The best i can recommend is the one by Brian Hays at Impact : https://www.impactgs.co.uk
Many outline libraries you find online, around Europe and the states tend not to have as many UK vehicles and are not as accurate.
Taking Pictures of vehicles and scaling them up is a good way also, I agree!
However, this isn’t always convenient for the customer and the pictures have to be taken properly to get true sizing.
You also have to make sure the vans clean, which again isn’t great when it turns up dirty just for this purpose of the picture, then come back to be fitted.
That said, i do use this method for machines, JCB, buses etc but this is normally because We visit them at their premises.
(yes the vehicle outline library’s do have these on file also. I mean for those that are not on it.