What are you doing? are there already windows in the rear that you want to match more windows up to, or are you starting with a panel van with no windows and are cutting holes to make some?
If the latter, then just find any, be they from Just Kampers, Camper Glass, Leisure Van Windows or any of the rest.
Do you want (or want rid of) opening sliders that leak? Good luck with that.
If you have no existing rear windows, then it's easy, just get all of them from the same place at whatever tint you require, at whatever price you're willing to pay, they don't need to match the fronts (unless you want them to be almost clear)
If you already have side windows then you need to look at the existing glass markings to see if you can see who made them, then try to match from there or bite the bullet and remove the existing and replace the lot with matching glass from elsewhere. This could be cheaper than trying to match VW or a specific pattern glass like Bebb from Salop Vans.
Rear windows the same, I tinted my VW original to match the Leisure Van Windows/AM Glass sides but it may be just as cheap for you to replace the lot.