Ah yes they are the old type. I have a feeling they are not to UK safety standards anymore so Bulgin stopped making them in preference to the standard IEC (kettle) connector.
My issue with them was they were made from bakelite and the cable socket back cover would break exposing live terminals. Also the male pins in the panel mount socket would come loose, leading to all kinds of bad things.
I assume you are doing a restoration and wanting to keep the looks/spec. If possible do consider using a modern IEC connector, they are much safer
Other than that maybe there's some Chinese copy in ebay. There are ex-equipment ones but... safety risk.