They do do a slider for the rear N/S. I saw them at Custom and Commercial a few years back. I've not seen it though on their catalog.
Do you mean the opening 1/4 window (usually one on most busses e.g. Westy but 3 on a deluxe microbus)
No it a sliding window http://www.justkampers.com/shop/t2-bay-window/original-parts/windows-seals/237845353-nearside-rear-sliding-window-j16870.html. Got it on back order now. I find it gets really stuffy in the humid weather. Plus sometimes need a bit of fresh air in the morning if you know what I mean.