There are a few people selling them on eBay (at a slight discount, actually). Or, if you have a credit or debit card, you can pay right on the Straight Talk site - either every month (or few months, you can choose how many months you want to pay for, just like the cards) or set it up so they debit the money every month automatically.
If you mean a brick and mortar store, none that I would trust with my money. (Joe's Discount Cellphone Cards doesn't sound like a guy I'd want to do business with.) There's some sort of tie-in between Walmart and American Movil (ST's parent company), so I don't think you'll find any B&M stores selling the cards.