Isn't that an Edison base (lamp) socket size?
I am not really sure where you find one but I would start googling "antique electrical" and see what you find.
There are some restoration places that might even have that cord. I haven't really done much personally but when they were renovating
the Ringling Ca D Zan mansion in Sarasota there was a guy who turned up some pretty cool stuff.
I looked a little
Maybe this guy can help you.
http://hudsonscustommachining.com/