Answer 1

They are held by wild Trubbish and wild Garbodor.
5% chance for Trubbish.
50% for Garbodor.

Answer 2

In Black and White, there is a 5% chance that a Trubbish, Croagunk, or Toxicroak will be holding Black Sludge and a 50% chance that Garbodor will be holding it.

Answer 3

True, they are held by Trubbish and Garbodor, but the percentage is incorrect. It is a 50% chance. My source is Bulbapedia.

Comments

Wrong.
http://bulbapedia.bulbagarden.net/wiki/Trubbish_(Pok%C3%A9mon) 5%

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I just looked at Garbodor, it said 50%

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For Trubbish, I mean.

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That explains the 1% Nugget