Please help. Emotional Support Animal information needed!

Hi everyone, I have been given a recommendation letter/note from my psychologist to seek an Emotional Support Dog. I honestly have no idea what the next step is from this point. Do I adopt a dog from a business that specializes in service animals or emotional support dogs? Or rescue a dog from a shelter and get it licensed?

Apologies for asking so many questions, but lastly I have seen many websites claiming to be the "official" dog registry trying to sell vests and ID cards. Does anyone know how legitimate these are?

Thanks for any and all answers