In the US system, HRT is a prescription drug, which means a therapist generally cannot prescribe it themselves - they have to work with someone who can, an MD or DO or someone else duly trained and licensed.
Lest that sound too discouraging, it's exactly the same legal situation with anti-depressants, and those certainly get prescribed no problem. Any halfway decent gender therapist should be able to refer you to a doctor for HRT (or several), so you shouldn't have to worry about finding one when the time comes.
If you start with an endo or general physician, they'll almost certainly refer you to a therapist - professional and liability reasons. (Technically they don't have to, but it's their medical license and liability on the line.)
If you have a favorite doctor already (who's sympathetic) there shouldn't be an obligation to switch.
Any other rules are the invention of your healthcare professionals, not what the law requires.