Surely most retailers only carry a fraction of the UPCs as Walmart does. Amazon beats Walmart, but who else? Wikipedia estimates 120,000 items per Walmart location and 35 million sold by Walmarts online. In comparison, McMaster-Carr has 570,000 products. Walmart is huge.
My experience buying off-brand Cheerios is that the texture is very different: much crunchier. They're one of the few products where I don't just buy the cheap store brand version.