I've found them in two places. One is a fiberglass place that seems to specialize in GM stuff and I heard their quality is way less than stellar.
VFN Fiberglass has an entire rear section in race weight. Don't know the quality, but I thought of maybe getting one of those and cutting off what I don't need. We'll see. I've heard NOS ones, if found, seem to fall apart in a couple years. So not going that route.