The Simple Key in Hollow Knight is a single-use item used to unlock simple locks at specific locations. You can find Simple Keys in several places:

1. Buy one from the shopkeeper Sly in Dirtmouth for 950 Geo.
2. In the City of Tears, in a room at the top right, just below the City Storerooms stag station.
3. In the Ancient Basin area: after reaching a location where you encounter the third Mawlurk on the ceiling, drop down a passage inside the mouth of a large dead Mawlurk-like creature to find a secret room with a Simple Key.
4. Inside the Colosseum of Fools: behind a breakable wall on the right side outside the arena, by defeating the Pale Lurker enemy.

These keys can be used to unlock:

• The entrance to the Royal Waterways from the City of Tears.
• The cave to the right of Dirtmouth beyond the graveyard.
• The Pleasure House in the City of Tears.
• The Godseeker's cocoon in the Junk Pit, which grants access to Godhome.

Each Simple Key breaks after one use.

In summary, the simplest way to get a Simple Key is to purchase one from shopkeeper Sly or explore the specified locations to find keys in chests or by defeating enemies. Then use them to unlock the associated simple lock doors or entrances.