How to find businesses running their operations on spreadsheets?

Isn't it most of them? Look them up on LinkedIn and ask. Walk in the front door and ask.

In my experience, most businesses are running some portion of their operations in spreadsheets.

More importantly: they don't want to change. They're running things out of spreadsheets because that represents the technical skills their staff has. They don't want to learn a new, specialized tool for a process when a spreadsheet works. They don't want to pay for that specialized tool. They don't want to deal with troubleshooting it when it fails.