How and where can I find the keys for the locked boss rooms?

Most of the keys are late game items, so if you're not past 60-80% castle exploration, these will probably contain spoilers

• Celeste's Key - Hall of Termination
  • When you exit the teleporter room, head straight up and then open a secret room in the ceiling
• Carpenter's Key - Dian Cécht Cathedral
  • At the top of the right-most spire
  • Requires the Invert shard
• Millionaire's Key - Behemoth's Den
  • From the entrance head left, then down, then right. Near the top of this room is a chest on a ledge that you need Invert or High Jump to reach
• Warhorse's Key - Glacial Tomb
  • From the teleporter, head as far left as you can. A Gusion Cannon is obscuring the chest in the left-most room.