Receive the following fishing rods from the corresponding people:
WHERE TO OBTAIN
Obtained from a fisherman on a jetty on Route 209.
Obtained from a fisherman in Jubilife City.
After defeating the Elite Four, go to Snow Point City. Your rival will invite you. Take the boat to the fight area. As soon as you arrive, go up and there will be a fisherman. He will ask you a question, agree with him and he will give you the Super Rod.