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If the grocery store supply chain falls apart, it is unlikely you will be able to get any food from afar for the same reason (or, at very least, the people normally performing the deliveries will be too hungry to provide delivery service). Local farms are going to want to hang onto their production for themselves now that they have no other food source.

Realistically, that leaves the food you are growing yourself. 1/3 of your normal intake does sound like a struggle already, but to compound the difficulties, you have to ensure that you have the food available when you need it which is more complex than setting up a garden. Not impossible, but certainly a lost art. I'm not sure it really is feasible to expect many to follow through with this.