You purchase it from any of the vending machines on the Celadon Department Store's roof. You only need one.
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The place where it flashes is where you can find the Pokemon It would flash blue if it was found in the water
you have to find it
In the Pokemon forest
Water Stone...Well... If I remember correctly, there's a guy who's willing to trade a water stone for a Blue Shard. Oh! And you can find one on the Abandoned Ship.
in water