I'm in the same boat as you. Every time it rains the slider channel inside the van fills with water! I've got some very healthy looking moss growing in there at the moment.
Let me know if you find a source of replacements.
sell all kinds of seals on a per metre basis, and i remember i got some spongy rubber strip about a few cm thick that had a adhesive backing ? maybe this could seal up any gaps & allow the window to slide ? im guessing there are places like this in every town.
I get the same trouble with mine, i reduced a lot of it by cleaning as far as poss the funny fuzzy strips that fill up with gunk, allowing the water to capilliarise (?) through. It hasn't solved it by any means tho, now i have got some rubber U shaped strip and put it down the back edge of the slider glass. It stops a lot of the water blowing in but still hasn't made it water tight.
Thinking head still on.