How do you get a super rod in Pokemon white?

You can't get a Super Rod in Pokemon White. The Super Rod was introduced in Pokemon Black 2 and White 2, not in the original Pokemon White.

In the original Pokemon White, the best fishing rod you can get is the Good Rod, which you can obtain from the fisherman at the Undella Bay.

The Good Rod lets you catch most Pokemon in the game, including some rare ones. It's a powerful tool for any trainer, and you should be able to catch everything you need with it.