Woodland Mansion Finder
Edition
Version
Seed
Mansions sit on a grid of 80-chunk regions and only generate in a dark forest, so many regions have none. The nearest one is often thousands of blocks out.
Evokers drop a Totem of Undying when they die, and woodland mansions (and raids) are where evokers are.
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.