You can buy food-grade buckets new or used. New ones typically cost $5 to $9 for the 5-gallon size; used ones are anywhere from free to $2. If you buy them new, be aware that sometimes the posted price is for both the bucket and lid, while sometimes the bucket and lid are sold (and priced!) separately.
I get all of my buckets from bakeries; usually it’s the bakery inside whichever grocery store I'm at. When I was in college, I got buckets from the cafeteria kitchen.
Bakeries get their frostings, fillings, and buttery spreads in buckets, then throw out or recycle the empty buckets afterwards. These things come in 2.5 gallon and/or 5 gallon buckets. Some bakeries get both sizes; others prefer one size over the other. Ask. The people there are usually willing to collect buckets over a day or two for you to pick up later. Be sure to ask them to save the lids with the buckets.
Some bakeries give them away; others charge $1 or $2. Either way, plan on giving them a wash with soapy water once you're home.
Sometimes people who collect them to resell will post an ad in the local classifieds. I've seen them in quantity that way, for $2 apiece, clean and delivered if you want enough.
Locally, I've seen new food-grade buckets for sale at Smith and Edwards, Macey's, Kitchen Kneads, and the Bosch Kitchen Centers.
For those who aren't in Utah, that translates as a farm supply store, a grocery store (not a department store), and two kitchen specialty stores.
Whichever place you get your buckets from, be sure the buckets are clean and DRY before putting food in them. Make sure the lid is securely on, and don’t store buckets on bare cement. Moisture vapor will get into your bucket. Put a layer of something down first—wood, a strip of old carpet or cardboard. See more tips on protecting your buckets and other food storage here.
HOW MUCH DOES A BUCKET HOLD?
A five-gallon bucket holds between 15 and 30 pounds, depending on what it is.
Oats are light; only 15 pounds will fit in. A 25-pound bag of flour can be convinced to fit, with a lot of tapping the bucket on the floor to compact the flour. And if you have dense things like beans, wheat, rice, or sugar, 30 pounds will fit.
A 2.5-gallon bucket holds half as much. (BTW, you can fit 11 (10-11 oz) bags of chocolate chips in this size.) I mostly prefer this smaller bucket for things I use less of, or things that come in smaller packages—like dried fruit or chocolate chips. You get an extra layer of protection, with a container big enough to be useful, but small enough to not have to dig to the bottom.
Six-gallon buckets are a common size, too. You can fit 40 pounds of wheat or other dense food in one.