There's no such thing as an "official" herbalist practitioner in the U.S. You could, conceivably (but not recommended), get a paper certificate from a weekend course and then hang up your sign for business. There are a lot of ways to learn, and you don't necessarily have to go to a school. But a serious herbalist will study for many, many years (all their lives, really), and include in their training clinical oversight and mentoring. Here are four schools as examples.

International Integrative Educational Institute

Eclectic School of Herbal Medicine

Northeast School of Botanical Medicine

The Herbal Academy