How to Find Grimer in Pokemon Pearl

Travel to the southern area of Route 212.

Step into a wild grass area. Wild grass is typically taller than normal grass.

Activate the "Poke Radar" from your list of items. Items can be accessed by pressing "Y" to bring up the main menu then clicking on "Items." The grass will begin shaking. The grass shakes harder when concealing a rare Pokemon.

Travel to the area of the grass that shakes the hardest. This will begin a Pokemon encounter. The only way to end this battle is by causing the Pokemon to faint or by catching it.

Encounter areas that shake wildly to eventually find Grimer. Since Grimer is a rare Pokemon, it will take several searches to find him.