I made this Ghost Mushroom farming guide to help out players who want to farm the herbs instead of buying them from the Auction House. It's recommended to have around Herbalism 245, so you won't get herb fragments. Ghost Mushroom grow exclusively underground in caves.
The Fungal Rock cave in Un'Goro is the best place to farm, it's the filled with Ghost Mushroom. You can usually get around 20 after one full run through in the cave.
The mushrooms will keep spawning until you picked every node in the cave, so go around in the cave until you can't see any new mushroom. Then you have to wait around 5-6 minutes until new mushrooms spawn.
If for some reason you can't farm at Un'Goro, you can go to the Funggor Cavern in Zangarmarsh. There can be only 3 Ghost Mushrooms up at one time in this cave, but the respawn rate seems to be pretty fast, around 2-3 minutes. So basically just pick 3 mushrooms, then wait for 2-3 minutes.