I'm sorry, but I have never had to look into that specialty because I don't have (badly enough) disordered sleep of the kind that a sleep specialist would help to treat. My sleep quality, even though it doesn't run very long each night (between 5.5 and 6.5 hrs typically), is rather good according to my Galaxy watch. I seem to get enough 'deep' sleep, enough REM, I only take between 10-15 minutes to fall asleep typically...and I enter each stage of sleep over the night within the range of time variation that the empirical data says is typical for healthy people. Even better, and no I'm not bragging...just very, very grateful...I still, at almost 73, don't have to rise even once to void. My 85 year-old dad has had to void at least twice each night for over 20 years...surprising.
I found this with the appropriate google search. It may be useful for your purposes:
https://www.highlyhealing.com/the-7-most-trusted-sleep-physicians-in-the-united-states/

Dr Antoniou at Penn Medicine. She does televisits as do the sleep therapists from Penn. have helped me tremendously