In Route 209 (east of Hearthome City)
Use it on any body of water to fish for wild water-type Pokémon. Speak to the fisherman at the dock on Route 209, just east of the Hearthome City gate. He will ask you if you agree that the good staff is really good. Answer yes to get the good staff.
Good Rod: Just outside of Hearthome City, near the entrance to Route 209. Talk to the nearby fisherman and you’ll unlock the good staff.
You must have obtained Professor Rowan’s National Dex and unlocked the Battle Zone. Once there, take a quick hike to Route 225 and you should see the fisherman almost immediately.
Feebas can only be found by using either the Good Rod given by the Fisherman east of Hearthome City at the Dock or the Super Rod given by the Fisherman in the Battle Zone after The Player obtains the National Dex. After players get their Feebas, they can level up by using Dry Poffins.
Pokémon Brilliant Diamond and Shining Pearl Trainers must beat the Elite Four, become champions, and unlock the National Pokedex by registering all creatures in the Sinnoh Pokedex. Once that is done, Dratini can be captured.
Fishing in Pokémon Brilliant Diamond and Shining Pearl is easy. Once you have a rod, approach a body of water and select the rod under Key Items in your bag to use it.