Hmmm. I thought there were sample letters in our Wiki, but I didn't find any.
Basically, the letter is 'To Whom It May Concern' and covers several points from the therapist:
Using both names (legal and trans, assuming you haven't changed your name legally yet), state how long she has been seeing you, that she diagnosed you as transsexual, and that you are ready to begin cross-gender hormone therapy. She should note that you understand the risks and the consequences of hormone therapy. She should also check (and mention) the WPATH Standards of Care for any other pertinent points.
You can go to any medical doctor or PA for hormones, although a good many would refuse you on either moral grounds or lack of experience. An endocrinologist may be best, if you can find one. There will be baseline and followup labwork related to liver function.
A TG support group in your area might know what doctors are trans friendly. Someone here might know if you mention your locale.
Robyn