When my windshield got broken by a seagull dropping a clam on it I had to make a template and a friend who works in a bus repair shop fabricated one out of plexiglass for me. It was good as new. Not sure that you can buy them as replacement parts.
There's a couple threads about this very topic, and in post #6 of this thread, SkunkBoat posted some great photos and a description of what he did. I did that same project for the stbd vent window on my 255, and mine came out great too.