Somebody makes lower corner repair pieces. l bought some for my dads California and welded them in. As far as the whole piece, nobody makes it. Like GS400 said, you'll need a donor car.
No, chevelle uses a different rear window, although likely some of the interior roof / sail panel / inner quarter and trunk divider pieces are the same. I would try "azchevs". Would suspect one could use sections from up through a 72 Skylark coupe as the rear window, so the channel was the same '68 to '72, to the best of my knowledge.
