Chuck,
When a part is no longer available we have two choices. Abandon the mechanism, in this case close and seal the window forever, or get the part made.
That part is apparently brass but it could be steel or aluminum and it would work just the same. Find a local small machine shop and ask them to make it for you. Whatever the cost I am sure it would be less than having Prevost remake the part.
Finding one it a junk yard for buses would likely be similar in difficulty to finding an honest politician because the number of seated coaches which do not have our type windows is very large compared to conversion and few conversions using that part are worn out. Maybe you can find one that burned or was in a serious wreck, but I bet it would be quicker, cheaper and easier to just make one.
If you are patient, and have a few simple tools you could probably make one yourself.