I don't think a therapist can write these letter. You need a fully qualified psychiatrist.
The WPATH Standards of Care have some information on this.
It also depends on the surgeon. Most require two letters, but in Thailand some require only one.
I believe its best in medial matters to get personal recommendations from other trans people as to who they would recommend. There's plenty of bad medical people around, and you won't be able to tell one from another otherwise. Either search for these or post your location and ask.