Dead Island Side Quests Walkthrough: Out of Stock
You get this quest in a bar in the Hotel Bungalow area.
There will be walker zombies guarding the area. Get rid of them. If your rage meter is full, now is a good time to use it to clear the area quickly.
Once the zombies have been cleared, enter the bar and find Stanley, who is pretty hungover. Talk to him through the opening. He wants you to bring him 5 bottles of booze.
Check your inventory. You might already have to enough bottles to complete the quest. Otherwise, search around the bars in the hotel to obtain the bottles he wants.
Once you have 5 bottles, return to Stanley and hand them to him. He will want more, so go look for additional bottles. After you give him 5 more, he asks for another 5 one last time. Overall you will probably give him 15 bottles of booze. Once this is done you have completed the quest. Talk to him to get your reward (a small xp and cash bonus).