A person could make just about any sign by downloading a picture and using Paint to add words. When the poster is just what you want, print it out on some quality paper and Plastic coat it. I stick my stickers and "special messages" to fridge magnet material. Fridge magnet can be bought in sheets at sign shops, possibly Staples and arts and crafts stores.
I don't like permanent stickers on anything of mine. By using the fridge magnet material I can move or change out my special messages.
I put those ugly dash plaques I get for attending car events on fridge magnet material.
I was at a friend's shop, his shop toilet has a bold sign on the lid suggesting, take careful aim before pulling the trigger.