To get a Simple Key in Hollow Knight: Silksong, there are four primary ways:

1. Buy from Pebb in Bone Bottom for 500 Rosaries. This is the easiest and fastest way early in the game, though it requires a significant amount of currency.
2. Defeat the Roachkeeper boss at the top of the tower in Sinner's Road to earn a Simple Key.
3. Find one on a corpse sitting on the eastern bench in Sands of Karak.
4. Buy from Jubilana in Songclave for 650 Rosaries. You must complete The Wandering Merchant quest to unlock her.

These keys are used to unlock special locked doors in different locations such as Bone Bottom, Deep Docks, Sinner's Road, and High Halls to open shortcuts or access rare items. So, the simplest way to get a Simple Key quickly is to buy one from Pebb in Bone Bottom for 500 Rosaries, but for game progression and variety, you can obtain them by defeating enemies or exploring specific areas.