This is a really good question as I just stocked up on vitamins recommended by my doctor but I never asked him about brands, and I know what is in the pill is damn important. I just don't know where to get it. I bought my stash from Swanson https://www.swansonvitamins.com/ a friend told me about but mainly because they have a great variety and low prices. I bought mostly their brand.

Thinking about this some more, I am going to look into reordering from 2 sources I personally trust, Dr. Mercola, https://www.mercolamarket.com/ and/or https://www.healthrangerstore.com/collections/vitamins-and-minerals as both seem to be about putting healthy things into your body to maximize health. And they both stress you must eat healthy, etc, but vitamin supplements of course can help particularly as you get older. Perhaps Swanson is a good source, and if I can find evidence of that, I'll keep using them, but these other 2 sources might be other good or better options.