"In times like these, you think people would pull together."
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The Supermarket can be farmed for Brand Champagne after the main quest portion has been completed. This requires the player to head back to the place where Joseph first took them to enter the Sewers. Follow that sewer to the Town Hall, and follow the second sewer entrance to the Supermarket. The Supermarket can be exited and re-entered to farm the Champagne from the coolers inside.
Possibly the best method for farming would be during the main quest.
When you enter the market take out the punks as normal, there are two coolers next to where you come in, go forwards and to the right are 6 more. Each can be farmed for Champagne, alcohol, and lemon juice. Continue right towards the plants and there is another cooler. Go back towards the six and then go forwards one more, then two more to the right by the large plant. To the left there is an electronics section, farm for Mod parts here. Continue your way to the back of the market, once you reach the far point you will see a small corrider with a door at the end. That door when you go near triggers a checkpoint. Open the door to the right and kill the walker inside there are a few energy drinks. The security room has one infected and a Walker, There is a metal case on top of the cupboards by the computers, and it contains a Savage Heavy Hammer.
The player gets a checkpoint upon entering the room with the survivors. In this room alone, there is a Medium Medkit spawn point and five coolers, which will randomly spawn Alcohol, Brand Champagne or Lemon Juice. If the player backtracks one room (which requires breaking through a door) they will find a Ram to the left around the corner, which can easily be killed for XP by letting it advance a few steps and blowing up the fire extinguisher at its feet. Behind the Ram, there is a Walker on the ground and a Machete spawn point with good base stats and one free upgrade already in place.
The player may choose to go back yet another room, though this can be risky. This small room contains a Walker and two Infected but also holds a Red metal chest and a Small Shotgun Ammo spawn point. Since the ammo respawns, the player can easily dispatch all three enemies using a Shotgun and then just replace their ammo from the spawn point. This can potentially be used to farm the Infected for Meat by aiming for the head and using the Shotgun's unique instant-kill ability.
Provided the player has not ventured past the room with the survivors, they can quit to the menu and then reload from the checkpoint in that room to farm all over again. Depending on how quickly the player can break down the door, and what they choose to loot, it should only take approximately 1 or 2 minutes per run to clear all three rooms.
One can return to the part of the final supermarket area where Wolf fires at you during Supermarket Journey and get a gun out of the metal chest.