As far as I know, new cab corner windows are not being produced. Used ones are available through Chevy Duty, etc. If you can find used ones with only minor scratches (that won't catch your finger nail) you could always get a glass polishing kit--Eastwood has one that I used to take minor pits and scratches out of the windshield of my '56. Takes some time but works as long as you don't have to buff too deep.
Good luck.
