You can either catch a Shellos or you can find a Gastrodon in water areas I think.
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Shellos evolves in lvl 30 in a Gastrodon.
Shellos evolves at level 30 into Gastrodon. It is the same on Diamond, Pearl and Platinum.
Gastrodon
You can use Pokemon like: Buizel, floatzel, gastrodon, and possibly finneon and lumineon