Any reason you don't make it yourself? Your profile shows you have a plotter.
Use frosted vinyl on either glass or acrylic. Your local plastics dealer or glass shop for the substrate and then apply the reverse cut graphics.
ORACAL 8810 Frosted Glass Cast Film
Is this the look you want? Try using non-glare acrylic - it has that matte look that you are describing, especially when using stand-offs.
View attachment 91133
this isgn looks to be made with P-95 acrylic, which is almost frosted.