Zombies can be found on roads, paths and around monuments. Roads and paths that have loot spawns have more zombies (less so on snow roads.) Zombies can be found in good concentrations above military tunnels, the grass areas of launch. They can also be found at ice lake and swamp perimeters. 
If you are not finding zombies they may have been killed recently in the area. Player placed turrets on scourge will target and kill zombies. You can place turrets to protect your shop visitors. Zombie spawns don't increase at night.