I'm not going to a supermarket, I'm going to IKEA. Generally just outside the cities, so no need to worry about mass overcrowding. They have food and hardware, enough to make weapons and barricades out of. Furthermore, in the two IKEAs that I've been to at least, are raised up a level off the ground - supported on pillars. They have one easily blockable stairway, and a lift. Easy to hold off. Plus, there was a kind of gallery down on the car park, so if there are infected milling around down there I can fashion traps and weapons and have fun. Or, if the water goes off, I can toss my waste down there. Then just sleep in a nice comfy IKEA bed, and wait for the infected to starve/decay/get killed by the army.