Walmart typically carries a large selection of supplements from the 'Nature Made' brand (yellow label), which tests/certifies most of their products through the independent 'U.S. Pharmacopeia (USP)'.
While that may not help with the arguments over the correct dosage, it at least means you can trust the ingredients and dosage on the label isn't a lie. A random mail order supplement or bulk powder seller on Amazon...not so much.
Fortunately, it's the former. It's a 200+ year old nonprofit founded by a group of physicians in 1820 that developed the modern drug standards that most of the world actually relies on.
Towards the bottom of the page at Delta's careers page you'll note that Delta's headquarters in Atlanta has a Walmart Pharmacy in it. Delta is essentially signalling to its constantly moving employees that you're more likely to find a Walmart near an airport than a CVS or a Walgreens (especially in super-rural America) and that pharmacy is a constant stream of pilots and flight attendants grabbing refills on their way through.
While that may not help with the arguments over the correct dosage, it at least means you can trust the ingredients and dosage on the label isn't a lie. A random mail order supplement or bulk powder seller on Amazon...not so much.