Id start off with a super walmart after HEAVILY blockading the main entrances. Youve got all the food, supplies, guns and ammo youll need for quite a long time. Plus with roof access and securely locked back doors your pretty safe and have an escape route..on top of the massive amounts of space in which to run around if they do get through. Plus you could hold up several friends/family with you and not be cooped up or really even hiding out.