Unfortunately the chance of finding a shiny Pokemon are quite slim, either your lucky and find one, or you trade with someone who has a shiny you want.
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you find the sihny ston in mewtoo's cave
Shiny Poliwag can be anywhere in water, but you would have a very very very little chance of finding it.
Randomly when you run into a wild Pokemon it may have a different coloration that's a shiny Pokemon. Also a free shiny Pokemon is found in the lake of rage.
pogi
it is very hard. you have to get very lucky or use a cheat