I had my assessment done when I was a child at a specialized clinic(?) Not quite sure what to call it. From my understanding, all my parents had to do was request the assessment and the rest is history. Not sure where you'd go since you're in the US, so this isn't entirely helpful, but I went to a clinic called Minds & Hearts. If you look it up, it might give you an idea on the kind of place you should be looking for.
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-- Logan