Well, you ca buy garbage vitamins anywhere, but I recommend a high-quality brand. You stand more of a chance of getting what you pay for. I get them at my wholesale club. Remember to be choosy about dosage, too, or your body just flushes out the excess.
Supermarket - I usually buy Nature's Bounty or Sundown - they have a sale almost every other week - half price. I have also picked up here and there at Costco.
BJs wholesale club. Or I shop around for sales on more known brands...comparing Walmart to target to cvs to my local super market they all carry the same brands and someone always has a sale.