I’ve actually had decent luck with restaurant supply stores, but it really depends on what you’re after and if you’re willing to buy bigger quantities. Got a 25-pound bag of lentils last year for way less per pound than at the grocery store, and it keeps a long while if you transfer to buckets with tight lids. Even things like oats or big #10 cans of tomatoes can be good deals—just make sure you have the space and a plan for using them up before they go stale.
Discount bakery outlets can sometimes be a goldmine, too. I’ve scored whole wheat flour and dried fruit there for a fraction of regular prices, though it’s hit or miss. Another one folks overlook is gleaning or bartering with local farms—sometimes they’ll let you have “ugly” produce at steep discounts, which you can can or dry yourself.
Instant mashed potatoes are surprisingly handy, agree on that. How are folks storing their large amounts of dry goods? I use old food-grade buckets and mylar bags with oxygen absorbers, but curious if anyone’s got a cheaper setup.