the problem with walmart is that the front doors are glass. You'd have to move a lot of shit to adequately block them off. Costco, on the other hand, only has solid doors and those metal rollup doors. Those are perfect for keeping zombies out as they are impenetrable. Also, most costco stores also have gasoline now, so It would be easy to stock up on that ASAP for generators and the like once power eventually goes down.