Have you ever the of the word "trade" and "merchants"? Do you "buy"? There's a reason for "specialization" and "division of labor". It's why we're so wealthy, because of our economy's vast ability to make stuff and transport stuff. Utilize people's comparative advantage, we can produce more unit of goods than we would otherwise if we do not specialize and trade.
Trade is the stuff of civilization. We should encourage more of it.
Locally produced food is the most obvious, practical thing to buy because it is fresher an tastes better which should be compelling enough on it's own.
I'm not saying don't trade, I just think we should think about what we're buying a bit more. As Fugazi said, "Never mind what they're selling, it's what you're buying!" Heh.
if "Trade is the stuff of civilization" as you say, is it not critical that our trade reflects our personal beliefs?
If you don't like what certain large corporations are doing, don't give them your money. In many cases, as with food, an easy way to do this is to buy from local farmers directly or through local coops.