Ender gave you a lot of good information. You shouldn't need another letter if you are already on T. I would suspect whatever PCP you get will keep writing the scripts. You could contact one of the GA therapists at
www.wpath.org and they should be able to refer you to an Endo n ear you that works with trans people. Another idea is to contact an LGBT organization in your area or PFLAG. I really dont think you will have a hard time with your PCP. When you get the list of PCPs for yout insurance, contact each one of them and ask i f that doc will keep your script going. Pick that person to be your new PCP