In Route 36 from Arthur on Thursday.
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In the Granite Cave at Dewford town. It is not too hard to find it there.
No Pokemon evolves by hard stone. Hard stone just increases the power of rock type moves.
You can get the Hard Stone from Arthur, who is located on Route 36 on Thursdays.
You may find a leaf stone in Alexa forest or Viridan Forest. It is in one of those places. You will have to look hard for it though.
A hard stone is a hold item, to give to a Pokemon to make their rock (or ground)type moves stronger. It doesn't make any Pokemon evolve.