You can farm Black Sludge off of Grimer in ORAS. There is always a 5% chance that Grimer will hold the Black Sludge. Grimer can be found in Fiery Path.
Here is the probability of encountering Grimer in Fiery Path: http://pokemondb.net/location/hoenn-fiery-path
If you are unfamiliar with "farming" then read this:
Farming items means using a high level Pokemon that knows the move Thief (TM) to steal items from wild Pokemon. For Thief to be successful, the user cannot be holding a held item prior to using the move Thief. Remember to collect farmed items from your farmer every time you successfully steal an item. You will know when an item is stolen because a message will appear that signifies the theft of the foe's item. Note: You can steal trainer items in ORAS, but cannot keep them after the battle ends.