Being in a small city, I looked for someone who was within the area for the most part. It helped that I got recommendations from other people who specialize in trans issues to look into two therapists, one who gave me my T letter and got me on the road to starting my transition medically.
If you aren't sure where to look, start with Psychology Today and you can narrow the search down to therapists in your general area, and even get as specific as those who specialize in certain LGBTQ issues. If that doesn't work, try putting "gender therapists in *insert city here or zip code here*" into Google/Bing/Yahoo.