From what I understand, you are trying to help an SEO agency sell to other agencies that themselves sell to dental/medical companies?

It's tricky without knowing more details, since this question keeps coming to mind: Why is the SEO agency interested in white-labelling their services to agencies instead of selling direct to clients? It seems oddly specific and convoluted.

However taking the question at face value, I would say consider the following:

* There is likely a very small pool of agencies who specialize in medical/dental. How do you know the ones that do don't already offer SEO services?

* If you can pull together a list of agencies you think would make good leads, the simplest thing to do is just cold call and try to book a meeting with their owner or account director to show how you can white-label your services.

I wrote a blog post about agency lead generation for targetted niches: https://www.proposify.biz/blog/lead-gen-system hope it helps.

Feel free to book a call.