Since you are replacing the downstream sensor, doesn’t the new one come with it’s own connector? Or is the problem the corresponding connector on the wiring harness end has been removed for some reason, and is now missing? If that’s the case and a replacement harness connector is difficult to source, an auto electric shop could probably figure out a way to use a different connection method. They might decide is easier to remove the new sensor’s connector and just hard wire it to the harness by soldering, with no connector.