Hode Plank's can be gotten in Makna forest, from the bow-wielding Hodes. One of the best drop rate is from the Hode right before the turn into Hode lair, he's next to a green thing. Kill him, then save and reload, repeat until you have all the planks you need.
Hode Lair can be reached by going south, following the edge of the map, from the pollen windmill landmark.
You can trade it with Nelo in Frontier Village. He gives it to you for bonus.