Freetime Hobbies/Blue Ridge Models. Shaded numbers in several sizes as well as WWII unshaded in white and black. Freetime also sells a set of letters (shaded) and Command "E" award decals in 1:350
Since Hawk Graphics paused their production, these are my current recommendation. Hawk's low-viz/shaded numbers were a great choice for a more modern application but I don't know of another source