Prior to the fifth generation of Pokemon titles, the Fishing Rod was divided into three distinct categories: Old, Good, and Super. Each rod would allow you to catch different types and levels of Pokemon, with the third offering up far better fare than the other two. Surely the masters of Pokemon would not change this!
Well, actually, they did. The Fishing Rods of games past have been compressed into a single, powerful Super Rod in both Pokemon Black and White and Pokemon Black and White 2, and you need to beat the game to get both of them. This means having your character registered in the Pokemon Hall of Fame after defeating the champion who stands ahead of the Elite Four.
In Pokemon Black and White, you received the Fishing Rod automatically from Looker when the game restarted inNuvema Town - and, in fairness, you still get the rod there. In Pokemon Black and White 2, however, the rod is optionally given to you by Cedric, Professor Juniper's father, in her laboratory. Visit it after beating the final boss for the first time and you can fish to your heart's content.
Looking for the cream of the crop? Like Pokemon Black and White, the Super Rod in Pokemon Black and White 2 is a little different in that it can snag fully-evolved pokemon. All you need do is Surf out to whirlpools in water (of, if you're lucky enough, use ones from land) and cast your line. You'll likely get into a battle with a rare pokemon.