Before you go all nuts with food-grade buckets, realize that there is little you cannot can with a boiling water canner and a pressure canner, so keep canning. I've yet to get into pressure canning (which is what you'd need for meat), but every year, I teach myself to can another couple of things on my list. So far, I can can fruit, tomatoes, and applesauce. We put up a lot of tomatoes and applesauce this fall. I usually end up with more than a year's supply of applesauce this way, and end up giving some away to friends. Canning allows you to preserve things at the peak of freshness.
Also, utilize your freezer. It is really surprisingly easy to freeze your own fruits and vegetables. I need a bigger freezer, because I'm running out of space for all the stuff I like to freeze. For example, I have one shelf in my fridge that is all desserts--apple cake, zucchini bread, and cookie dough. I have another shelf that is all frozen vegetable stock. How do I make stock? I save the trimmings from clean vegetables--peels, ends, whatever--in a ziploc in my freezer. Obviously, the freezer isn't so useful for a survival situation with no power, but you'd be sitting on at least a month's worth of food, especially if you had a chest freezer in a cold location that you kept mostly closed.
Realize that some things cannot be stored for long periods of time without them turning rancid, specifically things that have any bran or natural oils left in them (like coarse cornmeal, whole wheat flours, steel cut oats, or oily nuts). They need to be in your freezer or fridge or somewhere cold.
Given the chances of spoilage from not storing things correctly, I honestly think it's better to buy some things in smaller amounts. There are a lot of nasty critters that can get into your food stash if you're not careful about it. Think about that--how much would it suck if you opened up your five-gallon bucket of flour to find it full of flour moths?
Root cellaring is another possibility depending on where you live.
Root Cellaring - Modern Homesteading - MOTHER EARTH NEWS - Root Cellaring
But to answer your question on food-grade buckets: you can buy them at a brew supply store. Here's a place online that has some:
Plastic Fermenters for Beer and Wine | HomeBrewIt.com Places don't "throw them away." Food grade buckets generally get reused, and reused, and reused by the restaurant who has them.