Since I had a long-standing relationship to a therapist (clinical psychologist, in this case) who knows me well already, she's the one I approached with this issue, and initially, after reading the SOC, she felt she didn't have the special training or experience in gender issues and would want to refer me out to someone else for purposes of the letter and all.
However, after three months of sessions in which we discussed my GID issues and my transitioning thoroughly and being satisfied that I met the criteria, and after she had a chance to read up a bit and confer with a colleague or two with more specific experience in GID issues, she decided that she'd be okay with doing the letter after all, which was fabulous for me because I would have had to pay another therapist out of pocket, and my financial resources are very limited.
Unless your situation is for some reason unusually complicated, I would think any experienced therapist who is willing to educate themselves a bit from the readily available information, or consult with someone for guidance, should be able to give you a hormone letter.
Anyway, the fact that I wasn't forced to go to a gender specialist saved me probably several months of time in my process and many hundreds of dollars. I'll have my letter in hand this week, and be able to start finding a doctor to get things going.
Obviously, if you're approaching therapists cold with this, you want to be careful, because as in many other professions, credentials alone do not a good therapist make. If I didn't know someone, I'd try to get references from other people.