If you want to find all four Simple Keys and their locks while playing Silksong, you'll have to visit various different biomes of the map.

One of the first items that might seem egregiously expensive in Hollow Knight: Silksong is the Simple Key. You’ll see this ...

Simple keys in Hollow Knight: Silksong are used to open doors scattered around Pharloom, but they're few and far between. As you're exploring Silksong, you've probably come across a few locked doors.