Trial Chamber Finder
Edition
Version
Seed
On Java they generate between Y −40 and Y −20, underground in almost every region of the world.
Trial chambers are everywhere — nearly one per region — so zoomed out they would cover the whole map. Zoom in and they appear.
On Java Edition, type /seed in chat (it needs cheats on, or operator rights on a server). On Bedrock, open the world's settings: the seed is listed under Game.