My old T, who I saw many years ago, referred me to my current T when she was unable to take me on as a client again. (so glad btw, as he's a much better T for me)
My current T gave me a referral for a marriage T when I needed one.

As far as a T that specializes in infertility issues, I know that the medical practice that I worked with on the infertility treatments had a list of several T that specialize in this area. So you may want to check with your doctors to see if they have any recommendations.