Supply Crates respawn regularly, and new Encounters and Infestations appear randomly on your map every once in a while, so just wait and you can takedown more Walkers as well as collect more loot!

The best way you can find more stuff is to keep Our World open and go out for a walk around your neighbourhood. As you walk your avatar on the map will move in relation to where you are, so go exploring familiar places to you and discover a new world full of Walkers out there!