How do professors find consulting projects?
I think there are endless possibilities and no definitive answer. So just to name a few:


A company rep sees a talk of the professor´s group and sees that they are a good fit.
A company rep reads an article of the professor´s group and sees that they are a good fit. (Which not only includes articles in academic journals.)
They know each other from former joint projects (like publicly funded ones with industry partners, ...).
A former student/colleague works for the company and knows the professor is a good fit.
They search online for professors/institues that fit their needs.
The professor worked for them before going into academia.
They get refered to him by other companies/institutes/individuals.
...