The package of drugs is an item you find in Bavelock. You can find it near the trapdoor leading down to the Thieves' Passage.

If you already talked to Rififi before finding the drugs, you automatically start the sidequest "Drugs are bad m'kay" when you pick them up and you can simply sell them to Rififi for 60 Pix.
If not then you get a text saying you are not sure who would want the drugs. Find Rififi and you can still sell them to him.