On Whirl Islands.
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You can find Seels all the time in the Whirl Islands
At SEAFOAM ISLANDS. There you can find Pokémon like SEEL and DEWGONG.
No you can't catch Mewtwo in Pokemon silver, you must trade it from Pokemon yellow.
You can get a Seel in Pokemon Pearl by finding it in the water on route 226.
There are a few places you can find a dewgong in on Pokemon Platinum version. You can find him on Victory road, route 226, and route 230. If you catch a Seel you can also get Dewgong by leveling Seel up to level 34.