My friend who owns (and rents out) her own studio only rents to people she knows, and the people they know.
If you are looking for any type of renters, as opposed to trainers only, she has successfully rented to high school dance teams and a fencing team to use her space on off-hours for their practices.
I put the word out in the community. I tell everyone I know that I am looking for a new trainer.
And I search for a trainer as if I were looking to hire one on sites like ACE, etc. Then contact those I find who I don’t already know personally (and a few that I haven’t seen in a while if applicable) to let them know that I am looking. I usually reach out casually with a simple “I am looking for a new trainer for my studio. Are you interested in talking about it? Do you know anyone that might be intereseted?”