It is not uncommon in houses to put in rectangular windows, but use whatever trim it takes to make the window appear to be the desired shape.
In my build I am using Andrew's hopper window design with nice big round corners on the out side, but with a rectangular window. Mine will open (and have screens) but they wouldn't have to open or have screens.
There probably is a dozen other ways to do this problem, besides buying a ready made window. My budget and inclination is to make it myself someway, even if it is more work than it might be worth.