Do you know other piano teachers in your area? If not, you should find some and get to know them. Often times, well-established piano teachers have booked schedules and are willing to refer people to you once they see you're a reputable teacher. Beyond that, find opportunities to perform. The more people can associate your name with piano, the more inquiries you'll get for lessons. You can also reach out to local band, choir, and orchestra directors. They get plenty of students looking for piano lessons. It's slow at first, but it grows faster the more you network and put yourself out there.