You might want to try the following government website search page (childwelfare.gov). There are many support groups solely for adoptive parents.
National Foster Care & Adoption Directory Search
I'm surprised you're having trouble locating a therapist. In my experience it's been the opposite problem - I've found tons of therapists over the years who deal with adoptive parents, but not with birthparents.
Anyhoo, maybe childwelfare.gov can steer you in the right direction.