Some Pokémon have different variants. Usually, these depend on the regions and can completely alter the Pokémon; for example, the Nine-coded fire-type Pokémon has a variant form in the Alola region that transforms it into a dual ice / fairy type. However, these variants can also be purely cosmetic, as with Pokémon Shellos. Here's how to get your hands on an Eastern Sea or Western Sea shell.
Related: What are the shape differences of Shellos and Gastrodon in Pokémon Brilliant Diamond and Brilliant Pearl?
First, the main thing to keep in mind is that Shellos variants are purely aesthetic. An Eastern Sea and a Western Sea Shellos offer no advantages or disadvantages over the other. Western Sea Shells have a pink and white color scheme while Eastern Sea Shells have a blue and green color scheme. Your Pokédex can be completed by registering just one of the variants, so you only need to worry about the specific Shellos if you have a color preference or are looking to add it to your personal collection.
A Western Sea Shellos is located on the western side of Mount Coronet, the mountain range that divides the Sinnoh region, and an Eastern Sea Shellos is located on the eastern side. A great place to catch a Western Sea Shellos is Route 205, which you will cross on the way to the Evisopolis gym. An easy way to find an Easter Sea Shellos is on Route 213, a path you will follow after completing the Pastoria City Gym.