If you go with Big Box stores, choose one of the warehouse-type outfits (Sam's Club, Home Depot, Lowe's, etc.). You can climb up the racks and hide on top of them. Heck, in the Home Depot or Lowe's, you can use the Lumber to make catwalks and have quite a setup, 20' off the ground!
If you really want defensive structures, find a grain silo. Or better yet, find a grain mill complex; some of those are huge aggregations of dozens of silos. Not a lot in the way of supplies, but impervious to almost any attack!
A National Guard armory is another good choice.
In Texas, the older county courthouses would be decent (they're more or less actual forts).
Most shopping malls would be terrible, owing to having way too many doors, and big ones at that--also note that most malls have lots of delivery doors and loading docs, too.
An office building might work pretty well, especially if it's kind of fancy and has a big foyer on the first floor (so that the first floor doesn't have a lot tiny little offices). Just block the stairs and the rest of the building's yours.