somewhere out there some place has em. i know cuz directron used to. i even ordered one when there was a sale. ussual price isn't that nice though, 19-29+
if you have the tools, doing it is rather easy. you don't get exact flushed results like bought windows, but plexi is cheap at home depot, and with alittle velcro or glue, u can just stick it on from behind. there are guides on the net.