When you are at the Troll Bridge, choose "look for another way around" or close to that. You will discover a cave. The FIRST time you go into that cave you will get 5 glowshrooms. After that you will randomly get a glowshroom when you visit the Lake of Purification in the Lizards Ruin.
If you already have glowshrooms in your inventory you can plant them in your house, although you need to have the big garden, when you collect you'll get ten of them.
