Plastic replacement windows are not readily available in the marketplace. Plastic can be temporarily placed in the window frame until a replacement window is installed. Though, as a permanent replacement, plastic windows would be impractical. Some convertibles have plastic back windows which can be replaced. These windows can be ordered from the local dealer or some auto parts stores. Side windows that roll up and down are almost always made of glass. Plastic would not be durable enough for side windows or windshields. It is much harder to clean and would need replacement frequently, as its clarity would be distorted when it gets dirty.
For more info, see Average Car Window Replacement Cost