You have to transfer it or you can trade for it.
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Happiny evolves into Chansey. It evolves with a oval stone in the day
Chansey > level up with max happiness > Blissey
One needs to increase Chansey's happiness to its maximum.
As far as I Know Chansey is not on the Pokemon Sapphire you can trade it from other versions like fire red or leaf green
Since you can't catch chansey you need to trade one from firered, leafgreen or Pokemon xd gale of darkness.