You can also go visit the witch and do her quest i think, she is the other one that can also allows you to be able to enter the cave when you get the item from her.
Which version are you playing? f your not playing the 13.0 version, which is the one you can get on Patreon, then you will have to wait until that version becomes avaible on here.
Glowshroom can be found randomly each time you visit the cave on the world map, not sure what the Witch requires to get the item needed to explore the cave.
