On this page of the game guide to Borderlands 2 you will find a detailed description of the side mission titled Rakkaholics Anonymous. You can start this quest in the Sanctuary location after completing the main mission named Wildlife Preservation.
2) Ellie's workshop
Accept the mission from Mordecai , use Fast Travel and move to The Dust. Inside Ellie's workshop , use Catch -a- Ride. Open the map and locate the vehicle from the above screenshot. It will be moving throughout the whole map and your task will be shooting the barrels on its roof.
After an accurate series, Kegs o'beer will fall down from the roof. Collect 10 of them and return to Sanctuary. Now you can decide who to give the alcohol to. Moxxi  will offer you a pistol and Mordecai  a sniper rifle.