Son in Law Adoption

My son in law was adopted in 1975. I know the name of his birthmother and the county where he was born. What is the easiest and least expensive way to find his birth mother. The couple who adopted him know her name but that is about all they know. I looked it up and there was a male born to the person on that date. My son in law and daughter would like to know his birthparents for medical reasons.