How can I get ground from a NEMA 5-15R wall socket without electricity?
I have seen adapters that are exactly what you want. On the side that plugs into the wall they have one conductive prong for the earth connection and two plastic dummy prongs to fit the live and neutral. On the other side they present the earth connection either as a single banana socket or a screw terminal.

So they do exist, but I'm not going to try to find a specific part number for you to order because shopping questions are off-topic. Keep hunting and try using "ESD" as a keyword since these are often used to hook up electrostatic discharge prevention equipment.