WILD HEARTS

Shortly after the start of the game you will build a couple of blocks (basic karakuri) to help scale a wall. Continue to progress until you come to a second wall, at the base of this wall is a tree and a rock that can be harvested for karakuri thread. The rock and tree respawn very quickly so you can harvest, spam blocks, harvest until you reach 200 or 1000. You will likely get this through natural gameplay but later rocks and trees have not respawned as fast as this first set for me.