In my playthrough,I think I possibly looted every ammo case in the map,cant seem to find anymore crates of ammo,so the only way I get ammo now is by the commerce provided by the library,dojo and sniper nest.It took hours but I finally get 22 cases of ammo,with -4 being the upkeep.Today I woke up and I only had 2 cases of ammo left.The survivors managed to use almost every ammo in one night.Now its going to take hours to get that ammo back because this absurd high upkeep(it was -4 not -20 daily).I checked the log and there were no random events in which survivors spend ammo.Im raging here,all those hours training strangers and library access for nothing because the game does not know he must save ammo and use as little ammo as possible.
You can have Lily search for ammo over the radio. It takes about 9 minutes of real time. Sometimes she doesnt find it but most of the time she does. It def helps so your not searching around the world endlessly. Besides that you'll find plenty once you get involved with the army later in the game.