Google, won't let me find these (1mm, 5mm length plug, stackable), linkably (but I CAN get the image).
They are probably out of stock, now. (It seems to link to a very out of date advertising site).
Possibly Rapid Electronics (UK), sold them, in the past.

EDIT: Could be 2mm diameter, or even 4mm. So, forget this.
Another option (such as
https://www.mouser.com/datasheet/2/516/BananaPlugs%20D7MOOCE-257160.pdf which mentions it), is to go for "Custom Patch-cord manufacture", depending on what budget and quantities you want.
Their experience would probably mean they know where and how to get the right plugs, and/or they can make them up. From empty plastic shells, and the range of connector bits and pieces, they have in stock.