SPOILERS AHEAD*****
The planks are part of a chain quest i think
when you get to the demon's city to the right of the door from inside you will see a man standing between two buildings but remember that the man wont appear if you JUST got into the city it will appear some time later not sure if you have to defeat gustav or not for it to appear
EDIT: my mistake i think you get them from a lumberjack in the barbarians town. If you speak to the goblin besides the orc in front of the inn from the demon town then he will give you something wich you will give to the lumberjack