I'm leaning more towards these now:
D & K Tanks, We are a supplier of quality Culverts, Pipe, Pressure Vessels, Carbon Steel Storage Tanks, Silos, Stainless Steel Tanks, Blowers, Air Locks, High Pressure Pumps both new and used and much more.
Steel
tanks with almost 2" thick walls. Most are for chemical or pressurized gasses and getting a 10-14' diameter one is still in the 4 digit price range with a couple of the largest ones running 10 grand. They won't have the structural weakness that the ISO shipping containers have and could be buried underground. Another 'habitrail' network of 10' drainage pipes could be used as well if you reinforce it with concrete for longevity. OR, you could just use small sections as hidey-holes with no long term amenities, but an adequate short term safe house/base for long range scouting/looting.
Yeah, I do a lot of research and keep changing or improving my 'ultimate' zombie plan. That's the major reason I haven't submitted an official ZPoTW.

I keep changing my mind as what would be the best one, they all have so many potential flaws!