I got mine here. https://www.ebay.com/itm/WW-II-Vehicle-Electrical-Outlet-used-on-any-vehicle-to-connect-to-a-trailer/264389039475?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2057872.m2749.l2649
I'm not sure if this is the correct one, but I saw it earlier: https://www.ebay.com/itm/Willys-MB-M38-GPW-G503-Trailer-Socket-Dodge-WC-CCKW-GMC-Half-Track/283540291440?hash=item4204517f70:g:KJYAAOSw-kdX1sMwCorey