Both of them are in the swamp area on the world map, the icon above the ruins when looking at the bottom-right most part of the icons, look around in the swamp until you unlock the dark version, after you do that, you should have the option to explore the Dark Swamp instead of a normal one in there, explore that until you find the Witch's Hut, if the box mention 3 purple ? then that will be her hut that you trigger, it can happen randomly whenevery it is day or night.