Ok so I think one of the best long term places to go (besides armories, I think gov/mil/police would take those) is a home improvement store like Lowes. There is a Lowes like 5 mins walking from my house, I used to work there so I know where to find keys. The building itself is concrete block. Outside is concrete block to block the glass doors with (there are 4). There are 2 forklifts (propane powered) they could be used to move materials or people to higher ground. The back of the building has a huge 2 story tall gate located in a huge bull pen, vehicles and people could get in/out there and if I'm lucky the big almost semi truck would be there. The garden area is all fenced and locked in, but could let someone in if needed. Lumber also has another large gate in the front. The store itself has huge 2 story shelves where people could live. It's like a 30 second run to a grocery store and a big 5 which carries mosins, sks and shotguns; as well as ammo for them. Did I mention all the tools (obviously hand) that could be used to secure the building and the essential building materials within. Ladders that go to the roof for lookout. I'd have to Send some people to kmart and walgreens for batteries; car and otherwise and propane tanks.
So remember this would be about pros and cons of a home improvement store.