Well, it's kind of horrible sounding, but a good place to scavenge ammo is off of dead guy you just shot. But it's actually more useful to just take his gun right along with the ammo. That way you have a spare. And something you know will work with the ammo you just scavenged (one would hope at least - it's possible that the dead guy finds himself in that state because he had the wrong ammo for his gun!)

Better yet, don't be an idiot and plan on scavenging ammo in the first place. Have enough on hand to get the job done from your private stock.

p.s. - You should have your own "spare" guns too, so you don't have to depend on the dead guy supply route.

WalMart will be stripped of anything useful within 3.7 nanoseconds of the start of a disaster, no need to even try looking for ammo there...