I think you'll find they are generally entirely custom, usually with a thin PCB that has the plug contacts as pads on one side and most of the circuitry on the other, with an injection moulded spacer between the PCB and the shell to hold the contacts at the right height so the components don't get crushed. No doubt there is some sheet steel stamping and forming manufacturer, probably in Shenzhen, knocking out the shells for many brands, but good luck finding them unless you've got Cantonese speaking 'boots on the ground'!