Hello, I work with Digital marketers as their sales consultant. Most of them do PPC ads, AdWords, or generate leads through Facebook ads. They do pay per lead but they also work with clients one on one with a fixed rate.
For example, I have worked with some that are generating Chiro, plastic surgery, and law-related leads, and what they do is to drive calls to their business but will charge the business per lead.

Again it depends, you can also work with companies that will pay you to help them monthly on a fixed rate instead of a conversion basis.