If you are looking to raise your visibility by appearing as a guest on podcasts, one approach to getting booked is to do the work yourself: identify podcasts that could be a good fit, reach out to the hosts, and pitch yourself as a guest.
A more expensive but more efficient approach is to hire a podcast booking agency, such as Interview Valet. Those agencies typically charge a recurring monthly fee that works out to about $400 per podcast booking.
Another option which I just learned about from Rick Watson is podcastguests.com, which connects podcast guests and hosts. He has gotten booked on four podcasts by using the site, which costs just $99 per year for a basic subscription to be listed as an expert, or $299 for a premium subscription.