A local TG support group, LGBT and especially T specific organizations are a good places to start your search. Nothing like first hand reviews. You might also look to see if there is a local PFLAG chapter. Many have a good trans outreach and would likely have a few recommendations. Best is if you are near a big city, there may be an LGBT or gender clinic.
When all else fails there is the Psychology Today Doc-Find used with a lot of caution. They do have a "Gender Issues" category. Many times the provider seems to feel just having a class one day about trans counts towards being qualified. If just LGBT, Gender Issues, relationships and anxiety/depression are checked, likely they are a for real gender therapist.
You may need to make a few calls before finding someone. Most therapist will spend a few minutes on the phone with you for some Q&A so both can see if the other is a fit