It doesn’t make any difference whether you buy an iPad from WalMart, Best Buy, the Apple Store or other retail outlets, they will be the same. The biggest difference in buying an iPad from WalMart over the Apple Store is that you will probably get a much better price, and you can still get the warranty and take advantage of the AppleCare program if you choose.
If buying them from somewhere else though, such as eBay or Craigslist you might want to make sure they are not refurbished models unless you don’t mind taking the risk.