Best chance for something on Walmart etc. is to make sure it's sold by Walmart (or whichever store) itself, and not a third party listing on their site. Walmart stores in person I've found reliable for non-counterfeit, but online can be less reliable. Same for other stores like Best Buy. I always check if there's a "sold and shipped by [name]" button in their filters, and always apply those, so I know it's something that's been at least hopefully looked at by the retailer.

Some things it's just better to go to the manufacturer's site directly. Ordering a laptop battery from Best Buy's site can get you some knockoff that has sat losing life in a warehouse for the last two years, already nigh unusable by the time you buy it, for example (and a sadly true example).