Welcome to the forum Joe! There's lots of old parks in St. Louis that you can metal detect at. Some parks might require a permit, so definitely check online first. Your best option is to just door knock, or ask your neighbors and friends for permission to metal detect their yards. Be sure to dig clean holes so that you can cap the ground close to how it was before you got there.
There's two clubs that are in your area that you can join. One is the Gateway Metal Detecting Club https://gatewaymetaldetectingclub.com/