Is there a directory of such clinics in the US? I'm working on a product for clinics and need to assess the market size.

As a mental health professional with 20+ years experience, now retired, I would wonder your exact definition of "clinic" and whether you're talking private as well as public ones. I'd feel your State governments would have a list of all the public ones, but I also think that the various licensing bodies in the State capitals would have such information. I know that's not very exact information, and things have changed in the practice over the years I was in the profession, and having retired about 9 years ago, I don't feel I know the titles of the agencies or licensing bodies you'd want to talk to, but I bet a call to a professional organization in your area would get you the name of such licensing bodies and they might have a State roster of private and public clinics, maybe even nationally. Remember, however, that your definition of "clinic", and "mental health" may influence the final number you come up with because some clinics treat substance abuse / dependence, and some do not, and some are inpatient and some are outpatient only. But I bet if you call a local professional, they can get you on the road to your answer pretty quickly. God Bless you.