It's funny that you say that. Over the years, I've found that Walmart provides the best price and since it's a seasonal item, the salt there tends to be fresher and cleaner. Is there hard data to support this? No. It's anecdotal. When I went to HD tonight, their stock of Diamond Crystal looked gross, and felt hard and clumpy. You could tell that it was old and likely weathered. Ironically, it was a few bucks cheaper than the water softener pellets.
While brand matters (stay away from Clorox at all costs), even "good brands" have bad lots, and poorly stored inventory. I'll likely do exactly what you suggested and head to a different HD or Lowes that has a higher churn rate and a likely fresher supply.