Just search the web for ActivityPub relay, you might add github.com as the site because someone has gathered a list of known AP relays there.

But beware that once you start adding relays you'll get a flood of messages in the federated timeline and much of it will be Japanese or other asian languages. Which might scare potential users away. So just silence them one by one, refuse media files from them. People can still follow users on these instances but they won't clutter your federated timeline.