Sliding windows... spare parts?

[Camperdan]

Always fancied some fresh air in the back of my bus - so had my eyes peeled for a good second hand sliding window as I have always like this style.
Just so happens that at the Volksworld show I found myself a matching pair for the opposing sides... so treated myself!:

There in pretty good nick and with a little elbow grease should polish up nice.
I did notice however that one of the windows has a nylon bush in the runner of the sliding glass frame:

This seems to help with the sliding movement and also just lifts the glass slightly helping the latch lock tighter into the various cut-outs for the various closed and open positions.

Does anyone know anywhere that may sell replacement parts for these windows, or do any of you happen to have any bits for these? As I am certain that the frame should have 2 of these each - one at each lower end of the sliding frame.
Here's a few close ups of the one that I have removed:

And here is the other end where I'm certain another one should be:

Any ideas or advice greatly received..... Thanks!