Sly's Shop (Dirtmouth): 4 shards can be bought here for Geo (150, 500, 800, and 1,500) after finding the Shopkeeper's Key and Bretta's House in the room under her house.
Forgotten Crossroads: defeat the Brooding Mawlek in the top-left area, Lower section near Goams (needs Mantis Claw), Rescue 5 Grubs (Grubfather).
Greenpath: In the Stone Sanctuary, found inside a room with No Eyes.
Queen's Station: On a platform in the upper-left area.
Royal Waterways: Behind a breakable wall in the west section (needs Simple Key).
Crystal Peak: Above one of the dark tunnels/benches and have to fight a boss.
The Hive: Breakable wall on the left side, requires a Hive Guardian to smash it.
Deepnest: located in the Fungal Core area, near the entrance to Deepnest
There are two Mask Shards located in the Resting Grounds in Hollow Knight: one rewarded by the Seer for gathering 1,500 Essence, and another obtained by completing the Grey Mourner's "Delicate Flower" quest.
