They are held by wild Trubbish and wild Garbodor.
5% chance for Trubbish.
50% for Garbodor.
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.
True, they are held by Trubbish and Garbodor, but the percentage is incorrect. It is a 50% chance. My source is Bulbapedia.