You must be talking about that neat looking 1978 Boler
-- you bought the one in Atlanta, right? Congrats!
Are the windows
I'm not sure why you need to replace them: Is it because they are broken? The trailer looked pretty nice in the photos, but..? They are really nifty, functional windows
(to my mind); maybe they could be repaired if they have problems? The "backframe gasket" that seals them to the camper is available from Vintage Trailer Supply, as is the gasket that goes around the oval part, and, I believe, the cranks as well.
Perhaps you just don't like them and/or want different functionality, or...
(Recommendations might change once we knew the reason(s).)
If they are working, and you remove them, please save them or offer them up for sale
- they are pretty much irreplaceable nowadays.
If you are going to buy new ones - I don't know of any new, ready-made square-cornered windows you can purchase for those locations (if anyone does, please pipe in!). All of the new ones I have seen have radiused corners (not just on the window part, but on the whole frame). These could be installed, but you would either have to make them larger (and so cut radii in the new openings - but watch that you don't run into furniture, rounded shell areas, or etc.); or keep the same size and fill in the shell opening's corners with fiberglass prior to installing them.
If you do go with the radiused ones in the same size, Scamp
buys in volume and had good prices when I checked, as Donna also mentioned.
Another idea is to check RV salvage places (if yours are irrepairably broken, and you want more of the same), as I've seen the same exact type of window in older "stick built" campers.