Wal-Mart has a terrible selection. They're also terrible pants that fit badly and wear out ridiculously fast. You'll spend far less, in the long run, on Levi's. And they have a much better selection, if you actually go to a Levi's store .
Department stores can be troublesome, they try to stock the most common sizes in the most popular styles of 8-10 different brands, so if you're hard to fit you can have trouble. Discount stores like Ross and Marshall's have the opposite problem - they have a lot of odd sizes, but not necessarily the one you want. Cheap stores like Wal-Mart, Target, and K-Mart have a variety of sizes, but each style only comes in a limited range of sizes (so they'll have one style/brand made for fat people, one for tall people, one for short skinny people...) But a brand outlet store, like an actual Levi's or American Eagle or the like, will typically have each of their brand's current styles in just about any size configuration a non-obese adult human can exist in.