LMC truck sells slider windows & parts but not the ones with the vent windows. I used to have a 78' with the same rear glass and I'm not aware of *anyone* who sells that glass or any of the parts for it. The only thing I can suggest is to use a window latch from one of the front vent windows since the latch itself is the same.
http://www.lmctruck.com/features/FC/FCYearsCovered.htm
Take good care of those windows because of something happens to them you're *not* replacing them, the best you'll be able to do is put in either a regular one-piece window or a slider window but the slider windows with vent windows aren't available anywhere which I think is a real shame because with all the windows open, especially the rear vent windows, you get a real tornado thru there while driving and that's great in the summer when you want to cool down. I loved having them and I'm sorry I can't find them anywhere to retrofit my current 79'.