Here's the locations of the cat food and how to pick them up.
Cat Food Locations In Fortnite
There are four total locations in Fortnite where you can find cat food, split between two towns. Two are in Retail Row, and two at Dirty Docks.
You only need two overall, so can achieve the quest by visiting just one location if that's easier for you.
Neither spot on the map is particularly popular at the moment, so the only people you're likely to encounter are those doing the same quest or others in the area.
How To Find And Pick Up The Cat Food In Fortnite
Once you land you need to know what you're looking for. Essentially a medium-large crate with a small try beside it. There should be a faint blue light emanating from the wooden tray, this is what you are picking up.
Inside the tray is tins of cat food, so go ahead and approach them and interact to pick them up.
The magic of Fortnite ensures you don't need to deliver these to Joey, instead he just receives them, and once you've picked two up, the quest is complete.
Once the quest is complete you'll receive 15,000 XP and will have completed all of Week 3's quests. Congrats! If you're looking for something else to do, why not visit The O2 via Fortnite Creative!