You can get wood from chopping trees, trees usually drop one piece of wood per energy however sometimes you will be awarded 2, if you're lucky. Chopping trees at neighbors' kingdoms gives wood to both the chopper and the owner.
Oak Tree: 10
Pine Tree: 8
Willow Tree: 6
Birch Tree: 6
You can also receive 5 wood each update from the Logging Camp (6 hour default frequency, can be improved to 3 hours by placing trees around).