This might more be an issue of where you search than what you search.
All of the usual suspects that do electronics carry them. Like Farnell, RS, Mouser, Digikey, Aliexpress,....
Keywords: "banana plug" or "4mm plug". You might want to drill down to "isolated" or "sheathed" or "shrouded".
Many brands carry them, like Staubli, Hirschmann, Mueller, Pomona, ...