Welcome to the forum Frank Discussion.
I think you're going to be hard-pushed to find anything that's been created specifically for Scribus. It would be a fairly niche template in a format that isn't quite an industry standard.
Scribus can open Adobe Illustrator documents - I don't know how well it does that - so you could try looking around for an AI template.
You might also try getting a PDF from somewhere like here
https://www.duplication.ca/printspecs/vinyl.htm and creating a Scribus document "around" the PDF but that might be a lot of fiddly work.
Having said that, hopefully someone can prove me wrong about it being difficult to find one and has something they will share.