There are many Boss Spawners and different ways to obtain them.
Dread Lord Shrissa
Obtained: seen dropped in Cavern of the Afflicted
Obtained multiple locations
Pervak The Prime Mender
Obtained: "an energetic pacer" in The Hole
From Bosses of Norrath - Booster Pack
You can purchase a booster pack from the marketplace for 150 dungeon marks and receive ONE RANDOM boss spawner.