If you have the old ones you should be able to go to a glass shop that has plexi and have them cut new ones. If you have them in the trailer now, when you take them out they will be flat so you don't need to be looking for curved plexi, it bends to the shape of the body.
Roy is absolutely correct. The windows curve when installed. You can get new gasket and lockstrip from Scamp, or check locally. Don't use something that's just "good enough." You'll want to make sure your windows don't leak!
Ten Forward - 2014 Escape 5.0 TA
Double Yolk - 1988 16' Scamp Deluxe