Do you know who made the thing? Is it a Leer, ARE, etc., name brand? If so, the big guys always have the replacement parts, but hold your breath before you ask the price.
I know (unfortunately) a Leer Windoor for my F250 is $275 (I broke one), but it’s a mint truck.
I wouldn’t hesitate to replace one with a piece of plexiglass and then tint it, if I needed to. Right now, I have a really good spare ARE cap missing the back window. It was a $3500 cap like 25 years ago, and they still sell for 12 or 1500 bucks around here used. I’m going to use plexi and put a frame gridwork, or some expanded metal behind it for security, and sell it for more beer money.
I don’t know where you are, but around Atlanta you can pick up the little aluminum caps that fit the Rangers for $150-$250 in good shape. The fiberglass ones are maybe $100 more.