Having just searched, I'm amazed at how hard they are to get now, with the majority of searches telling me they are out of stock = probably not manufactured any more either.
You could carry on searching with the words, 'Holga cable release adapter' and maybe something will come up...
I'm giving some links to enable people to understand what you mean and to give you an ideas of what they used to cost:
https://www.amazon.co.uk/Holga-Shutt.../dp/B000XB43I6
https://shop.lomography.com/en/product-tokscc-name
And ahhh!!! I've found one seller at $27.20 + p/p:
http://holgamods.com/holgamods/Holgas.html
And secondly I can see how one of these would work:
https://www.srb-photographic.co.uk/s...ets-1463-c.asp
Terry S