The left and right rear quarters are available from KF Vintage, and you can get a beaver tail from ZCarDepot. You'd be missing the underlying structure and floor pieces. If you can find another shell for a good price, you might end up money ahead by time you're done, not to mention the time difference.
If you're attached to this shell, you could always find another Z in a junkyard and cut the back end off like someone did to yours.