Just normal gynecologists?
I don't see a way to answer this without powerleveling, so here you go:
My endocrinologist is at a woman's hospital, so I need referrals from a gynecologist to see her.
I simply looked up gynecologists on Google maps and called those who had conveniently located offices "Hi, my name is <name>, I'm trans and I need a referral for an endocrinological consultation at <clinic>. Is that something I could get an appointment for?" First one said they were fully booked for the next few weeks and could only accept emergencies, second one the assistant was a bit unsure but got me the doctor on the phone who was like "Sure, come in next week." So now she's my gynecologist.
No secret cabal of troon-friendly gynecologists, just the 21st-century version of the phone book.