Also, if your work or school or parents insurance covers mental health, then by all means I think you should attempt to find someone in your network, or even use your provider to find names and phone numbers. You can ask these therapists point blank whether they have experience with transgender people, or knowledge about them. Finding a therapist that works for you is sort of like dating, and is much better if insurance picks up the tab!
The thing that scares me about "online" therapists is that I wonder if local doctors will take their letters seriously, even if they have the right credentials.
It's just been my experience that there were no "gender" therapists nearby me, and the ones at a distance lacked the right qualifications or insurance network, and it seems I'm pretty lucky to have found someone close by me that knows what s/he's talking about and is helpful but doesn't "specialize" in gender stuff. Like you I'm looking for HRT, so I hope this works for me, but when it comes down to it, I'm going to someone for their knowledge, only their support and advocacy (HRT letter).
There's different ways of doing this, but if I were you, I'd just get in touch with as many licensed social workers and other therapists in your area, and anonymously explain what you are looking for.