I have that in my 2007 3075RL. But I have no idea where to find one for purchase. I googled RV pleated folding door, RV accordion door, RV pleated ceiling hung curtain and found some similar items. But nothing on a curving ceiling track like ours. But you might try googling like that and maybe call some of the manufacturers found with similar items and see if they do offer something to fit what you want.
United Shade, part of Dicor, offers something similar, but does not show a curved track. But a call might get a good answer.
http://www.unitedshade.com/catalog/p...pleated-doors/
I don't know how long ago Montana, or other brands, stopped offering the "open bath" floor plan with the privacy curtain. But you would think that replacements are available somewhere. You might try an RV parts store and see if they may be aware of such and if they could possibly order what you need.
Someone has mentioned before about an RV salvage yard, I think near Elkhart, that might be a possible source.