That person is printing them up on their own because that's what you sell if you sell on etsy but they are copying the style of a larger poster sized product which I believe originated in England.
I framed a bunch of the large ones for a store that was opening up but the store went out of business so I can't ask them about the source. They probably wouldn't have wanted to tell me anyway.
The style comes from British World War II propaganda posters. The crown is, of course, the royal crown and they would say things like "Keep Calm and Carry On." in that font style and sizing.