(I have changed the name of my pantry series to The Practical Pantry)
Where in the world are you supposed to find the food you are going to store? Good question, if only there was a simple answer. It's not really all that complex, there just isn't one perfect source. My food storage comes from a variety of places, some local and some not.
The first factor is considering what you eat, use your list to decide what to shop for. I try to stock foods in the most natural whole state. That means that I have buckets of wheat, bags of different kinds of rice, dry beans and staple ingredients like sugar and cocoa. For you that may mean storing flour, cans of tuna, and cake mixes. You need to store what you eat and eat what you store. One reason I store things in their whole state is that they last so much longer. Wheat berries will last forever, white flour for a couple of years and wheat flour for a few months. To use wheat though, you have to have a grinder so there are factors to storing whole food. Whatever works for you is the best for your family.
Most of my bulk food comes from a co-op called Country Life Natural Foods. You can mail order from them or get a group together to order. They will deliver to you if you have an order over a certain amount. I order every other month with a group in our area and just go to the drop-off to pick up my goodies. It is worth the effort to organize a group if you have several like-minded friends who would want to order every month. Another advantage is that you can split large amounts and save even more money. We are splitting 50 pounds of whole almonds this time. It's a significant savings over buying 5 pounds and everyone can buy the amount they want. Country Life also sells 5 and 1 gallon buckets and the Gamma Seal Lids.
My spices come from a local natural foods store that has a bulk spice section. Check around to see if you can buy bulk spices in your area. It is a great way to try a variety of spices at a super low price and you can buy as much as you need.
I buy my wheat from a local bakery that grinds their flour fresh every day and will sell you a 50 pound bag of wheat. Check with your local bakeries and see if they will sell you flour or wheat at a wholesale price. Our natural food store will order bulk for you when they place their orders and this would be a great option, especially if you have special diet needs like gluten-free or some other food allergy.
Some things I stock from a regular grocery store. Look for sales or coupons and buy enough to keep you until the next sale or longer. I found pasta we like for half price and bought everything they had on the shelf. If you eat a jar of applesauce every week and they are on sale for a dollar, don't buy 2 jars, buy 12 or as many as the store will let you. Sales usually follow a 2 or 3 month cycle so buy as much as you need to get you through to the next sale.
Most of these strategies require an initial chunk of money that may strain your grocery budget at first but will eventually make a huge difference in your budget. Several years ago I didn't have 20 extra dollars to buy 50 pounds of wheat but by cutting back on things like convenience foods, meat and dairy I was able rearrange things so that I could buy it. Now instead of buying a bag of wheat flour every week at the grocery store, I open my bucket and get out wheat to grind. It's not so much having extra money to build a pantry as it is making small changes to get to where you want to be. Believe me, having food stored away to feed my family is well worth any small sacrifices I made to make it happen!