Other survivor's loot
Why spend time looking in ambiguous places that might have supplies left over from before the apocalypse when you could just benefit from other people's effort?
They're not likely to be looted as they would only have been gathered in the weeks after the apocalypse.
There's a 50:50 chance they're already dead, in which case it's free real estate. If they're still alive, then that's a moral dilemma that's up to your group to handle....
[Edit] I realise I didn't actually suggest any physical locations these might be in!
The classic starting points would be to follow the passage of people from the start of the collapse - depending on how it all started in your world, people would likely be gathered by the government/military into refugee camps in order to protect them after cities become swarmed by the undead. These would likely have the best variety of supplies, as the government would have used the last of its coordination to stock them.
After that, it gets harder to specifically say where people may set up camp, or what level of supplies they'd be able to find. The longer term the settlement, the higher the chances of them having worthwhile supplies. If your survivors see anywhere fortified, there's a good chance the people who were/are in there have/had stuff worth staying there for.