To get a lot of arrows easily, find a relatively flat area with a few Bokoblin Archers on horseback, like south of the Tenah Ko'sah shrine in the Tabantha region. Get them to notice you, then angle the camera to a top-down view. If done right, the archers will shoot all around you, but never actually hit you. You can easily farm hundreds of arrows this way with no damage after a few in-game hours.