I cannibalised various GW decals for my Old Crow Future Wars fleet - you can buy decal sheets separately from their online shop.
Given that I've never taken much interest in Warhammer 40K, I don't really know what some of the symbols are supposed to represent, but there are sufficient different motifs there - as well as lots of numbers, letters and generic insignia (like lightning flashes and red triangles) of various shapes and sizes - to titivate a few non-specific sci-fi type vehicles.