The population would have to make forays out into the countryside to survive, or abandon the town, if not in a siege situation. Suburban dwellers have more vegetable plots and open spaces to provide foodstuffs. They would be less dependent upon shops. Those away from major centres are more likely to have their own food stocks because they cannot shop at will. But most of them will be reluctant if not dangerous to deal with.
Most families have some food in store.
It should be rationed and supplemented with whatever can be found. Storing food is a good habit to get into, especially if you live in an isolated place, which can become completely cut-off. If you have a year's food supply in store and add to it as you use it, you will not only be able to survive the worst, but will be able to live at last year's prices. But you will have to protect it. The stock does not have to be established in one go.