I know of no such forum that contains the information you're seeking. Most professional longarmers have their own websites where they list their pricing structure, examples of their work, their accreditation, etc.
However, for the other posters who are looking for longarm quilting forums as a resource for support, trouble-shooting, ideas, managing customers, and just show-n-tell, there are several groups on FB. I'm a member of a couple that are for longarms in general, plus a couple dedicated specifically to my machine.