Yes, it's pretty impossible to only buy locally sourced food, especially in the winter and maintain your sanity . . . our modern conveniences have virtually eliminated ways of preserving food that would enable us to have it over the winter.
I don't understand -- what modern conveniences have eliminated our ability to preserve foods? Every summer I garden (and buy locally for things like fruits) which I then preserve/freeze. I even packed my basil plants into layers of salt for the winter.
What is it that's preventing us from preserving our own foods these days? And preserving our sanity? Our ancestors ate only what was locally-available for generations . . . there
was no other chioce. So are you saying we should buy locally if it isn't too dull, but if it challenges us too much, give up and buy non-locally? Have it both ways?
It's not that simple. I have friends who have a big farm and grow organic veg here for large markets . . . their broccoli, etc. gets shipped on a yucky big plane all the way to Belgium, to sit in a yucky big factory there for a week or so, then sealed in yucky plastic, then sent back to the UK to be sold as "organic" in a British supermarket.
You'd actually be better off buying locally grown NON-organic stuff, as sad as that is.
What you're saying is that "some supermarket chains require the foods to be shipped and packaged elsewhere." That happens. But putting non-organics into your body is not the same as organics. I like the taste of organics, and I know that they're better for me. If I can get them locally, fine, but if not, I'm not eating chemicals and pesticides. These things are
not supposed to go into our bodies. But I'm saying this to a generation of people who eat fast food daily, according to research. (Do they know where all that fast food comes from?? It sure isn't grown locally! Even the Coca-Cola and ketchup packets are trucked/flown in!)
I guess to me it's about a lot more than pesticide free . . . I think food miles are just as important as pesticide free - as is eating as seasonally as possible.
Buying locally is a consideration, and it can be linked to organics . . . or not. You can get local or non-local organics.
Buying locally is, of course, the best thing all of us can do to reduce our ecologic footprint. There are lots of calculators online that you can do to test yours. (Most people would be shocked to find that if others lived like they do, we would need at least 3 or 4 planets called Earth to sustain all of us.)
Regarding food miles, it's surprising how many of those food-mile-conscious people don't even consider if the sofa they bought can be broken down and recycled, or how far it had to be transported. Some recycle a little, some nothing, others everything possible. (And do you walk your glass to the recycling center every day or two, as I do, or drive it there?
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Easy enough to drop it along the way when heading out shopping.) And do these same food-mile-conscious people buy their glossy magazines to read about organics (apparently no worries about the chemicals used in printing). And do they buy books, or borrow from a library? Do they tell themselves that they are buying local foods, but the ingredients that their local bakery uses are not local ingredients? How much new clothing do they buy each year? Where was it made? Where do they go on holiday each year? How many air miles are they using? (Dare I ask -- how many times did they and their S.O. fly back and forth over the pond before moving here?) Do they drive cars instead of using public transport exclusively (oh, but they are always one of the special few who are different from the rest of us who truly do
need a car
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)?
Point is, there's more to it than food miles.
I suppose it depends where your priorities lie. I find organic produce usually tastes better than the non-organic so if that's your priority you may not care where it comes from . . . If you are going to be strict about buying locally you'd better do it with everything otherwise what's the point, so no more imported goodies folks. 
Exactly my point . . .
EDITED to add: I'd better not see you 'food-mile-conscious people' posting on "Where can I get American food here?"
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Only kidding, of course!
Complex topic, isn't it though? Not trying in any way to provoke anyone -- just trying to point out all the aspects of it . . . there's more to it than food miles. It's 'everything' miles that matter in the big picture. Be aware of all of it.