Reddit reviews Rabbit Tanaka Doctor Who Tardis Police Box Tin Sign

https://www.amazon.com/Dr-Doctor-Tardis-Door-Sign/dp/B00LSPJKFC

Did you try Amazon?

Are you talking about the "POLICE [PUBLIC CALL] BOX" sign above the doors, or the "Police Telephone Free for Use of Public" on the door itself?

With the latter, in recent years it actually looks like the black lettering is in positive relief on the white panel, which is translucent material that can be backlit. If I wanted to do this and had the resources, I'd use some sort of white translucent acrylic panel, 3D print the black letters individually, and carefully set them onto the panel.

In previous incarnations, of course, it was a lot simpler, probably just paint, and that may be all you need to get the point across. I have one of those printed tin replicas of that sign, and it could probably pass as the "real thing" if I had a TARDIS replica to put it on.