Question

Where can I find a king stone in black/white?

Answer 1

I'm assuming you mean King's Rock.
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Icirrus City: Gift from chairman of the Pokémon Fan Club, if Pokémon shown has gained 99 levels since it was met in a Generation V game.

Route 13: From the Treasure Hunter. You have a 1/27 (3.7%) chance of getting a King's Rock

Pickup: If you walk around with a Pokemon with this ability between the levels of 11-30, you have a 1% chance of picking one up.
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Held by the following Wild Pokemon:

• Slowbro
• Poliwhirl
• Poliwrath
• Politoed
• Slowking

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Source: Bulbapedia

Answer 2

I assume you mean a King's Rock.

It can be found at Route 13 from the treasure hunter. Note that you will not get a King's Rock every time, and you can only get one item daily.

It can also be found 5% of the time on a wild Poliwhirl, Poliwrath, Slowbro, Slowking, Politoed, or Hariyama.

Source

Comments

The King's Rock is not on the ground at Route 13, it is given by the Treasure Hunter.

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Yeah, I noticed and edited just before you commented :]

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How did you notice? Your source is not specific.

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http://bulbapedia.bulbagarden.net/wiki/Unova_Route_13/
Looked here, trying to find a map to show where it is xD