In german we call it Werbegstaltung or official “Schilder- und Lichtreklamehersteller”.
For example this shop:
http://www.bohuslaw.de/
The dictionary says “advertising design” but I think this is too close to “advertising agency”.
Native speakers any ideas?
In the UK, they are usually just called a “sign shop”. Or “sign maker” or “sign writer”.
Checking taginfo, there is a few uses of shop=signs or shop=sign. Plus a few for craft=sign_maker or craft=sign.
I think a craft tag makes sense, because they are making custom products.