I just replaced mine yesterday. The must be plexiglass. The reason is the window is curve side to side and up and down.
sells the windows
and gaskets. Get the single piece gasket. It is possible to source the plastic local. Be prepared to have to sand and modify the fiberglass openings to accommodate new windows.
Also be prepared to pay to have installed.
Hope that helps.
I spent 150 for installation and 280 for materials