I think I have found the answer:
https://www.activegps.co.uk/garmin-zumo-590-automotive-power-cable.htm
The 590 and 595 share the same automotive (ie used in a car) mount, with (apparently) a unique male plug at the other end of the power lead that stretches back to the male cigarette lighter plug at the other end.
Scroll down the website in the link and you’ll find:
I have bought items from the people in the link before. They are very helpful.