I always liked the "Duck tail" spoiler on the Brite Smile group 4 car. It is very similar spoiler to the picture of the one you posted - wider though.
Here is link with a discussion looking for a "Duck tail" spoiler. Unfortunately, Iit doesn't look like they had any success finding it.
Brite Smile Group 4 car thread
PANTERA INTERNATIONAL
A DE TOMASO CAR CLUB
Presents the De Tomaso Forums
The On-Line Meeting Place for De Tomaso Owners and Enthusiasts From Around the World
Clicking on the banner will take you to the sponsor's website.