I was in radio for many decades...and I've never heard of any such system.
What you might want to do is approach a local station group and ask if you can work on commission only (is that what you meant?)....or, form your own "ad agency" so you can purchase advertising on any station that best-fits your clients. Just be careful about becoming legally responsible for invoices being paid by your clients. This hassle is the main reason I avoided running my own "advertising agency"...even though quite a few of my former radio accounts asked me to.
Glad I got totally out of radio ad sales into a much hotter field: on-line/internet.
Does that help at all? Hope so.