This always amazes me - in Aust, no matter where you are - there is always a healthy option....
Petrol stations always have skim milk, baskets of apples or fruit, bags of nuts (usually peanuts) and sometimes sandwiches too... if you are lucky they also have a 'mini supermarket' attached and will have cans of tuna, deli meat, and even vegetables... There is also corner stores in most places that will sell the same sorts of things... And, as it was said, when ever you hit a town supermarkets give the perfect opportunity to stock up on easy portables...
Sure - not perfect but you never have to worry too much...