I found the translation to "coroplast" in spanish and I did find some place to buy all over Spain.
MAgic word are:
Coroplast -> "Policarbonato de doble capa" or "Policarbonato de doble hoja"
I can buy how much I want in do-it.yourself hypermarkets like "Leroy&Merlin" or other stores or construction materials dealers. It's used for roofs, greenhouse and other.
I will need a lot of square meters for making my large van underbody
Maybe the golf ball vortex generators present on Toyota Prius and Volkswagen AG cars could be done by pressing some hot rolling massage machine or anything. Or maybe i will find "dimpled panels" (other magic word)
See the video:
HowStuffWorks Videos "MythBusters: Dimpled Car MiniMyth"