What custom software though? How does it get the data? What is it vulnerable to? What about the backend software, that's also part of the system? It's easy to say "it is hardware running software" but it obviously goes deeper than that.

I mean the easiest step would be finding someone that works with the software installing them and see if they like to talk.

If you're the kind of person that's good at making relationships you might even talk them out of old or dead hardware.

How would you find someone?

There are a surprising number of Linux thing running webstuff likely hacked together and shipped. I find a lot of vending machine computers on eBay that boot Ubuntu or whatever and start some sort of webservice thing.