The original duck tail spoilers were made by Direct Connection for the Duster/Demon/Dart Sports. The DC version was ABS and hasn’t been made in decades. They’re pretty thin material and prone to cracking, and generally sell for well over $200 if you can even find one. They were intended to be mounted with plastic push rivets, most people use metal screws and that cracks the pockets for the push rivets.
MAS made what I think is the best fiberglass version, but they’ve been out of business for probably close to a decade now as well. It’s very close in dimensions and shape to the DC version and the fiberglass work was decent. They're actually fairly thin, so they don't weigh a ton like some of the other reproductions.
Showcarsrob in Canada has had them for sale as well, I don' know if he currently has any listed on eBay. His version is about a 1/4” shorter on the width compared to the DC or MAS version, so it leaves a little bit of trunk lid exposed on the ends. It’s not a big difference and unless you compare it to a DC spoiler most folks wouldn’t notice. His fiberglass work is decent though, it’s probably the best version currently being made.
CNS on eBay makes one. The biggest thing about that one is I think they stopped the mold on the outer edges a bit too soon, so it has a vertical edge on the sides that makes it a bit shorter than the original DC spoilers. It's similar to the one by showcars in that respect. The fiberglass work isn't as good and it would take more finish work for sure.