You've hit on where post-apocalyptic fiction breaks down for me. There always seem to be unspecified sources of ammunition, shoes and gasoline. Black powder in makeshift muskets maybe, but a lot of PAF includes automatic weapons. People seem to scavenge resources from the old civilization way beyond the point you'd expect oxidation and putrefaction to have run it's course. There don't seem to be enough sick people or rotten teeth. People seem way too clean and too literate. In some climes, you'd need to spend a good quarter of your time chopping wood, which would probably not do much to advance the plot.

The roads and bridges often seem to be in much better shape than you'd expect un-maintained pavement to look after a few years.

I realize most of this stuff is produced for adolescents, but even so the standard of verisimilitude seems pretty low.