I am slightly unsure from your posts as to wether yo lost the Perspex window or the whole frame including window.
Two options occur to me,
1. If you have an exact match window, perhaps on the other side of the van, especially if it is a U shaped seating area, then these people can make a replacement Perspex window for you. Not used them my self but they are widely talked about as a way of getting an old window replaced although they need an original as a template.
https://eeco-ltd.co.uk/
2. We used to visit a caravan site each year at the same time as an elderly English gent in his Motorhome. His habitation door had been replaced by a upvc door with window and complete frame. It looked a little odd, but was a very functional way of keeping his van on the road. If you are not too fussy about appearances you might investigate this route as windows and frames can be made in all colours and sizes. It would require a little thought on how to secure the frame to the body work but not impossible, especially if it keeps your van on the road for a few years.
There are also a few classic Hymer Facebook pages were sources for older vans might be known of.
Davy