In any case, I am talking about the availability of items in response to the parent's obviously absurd implication "every food market I have been to sells X, therefore every food market sells X".

I am using food deserts as a counterexample since definitionally these are regions where certain items are hard to find. I know (hope?) that the person I am responding to likely doesn't believe every grocery store in the United States carries plain yogurt, but I also know that people here often forget that not every place enjoys the same level of choice that is enjoyed in places like the SF Bay Area

I truly don't understand why you are bringing up obesity, this feels very remote to what is being discussed.