Ways to get fire immunity?

[Dalebert]

Investiture of Fire, level 6 spell from Elemental Evil.

Efreeti Chainmail

What other ways can you think of? I know there are bajillions of ways to get resistance. Not interested in those. How can you get immunity?

[toapat]

kaladesh's Pyromancer Sorcerer upgrades its base Fire Resistance to Fire Immunity at lvl 18

[Squibsallotl]

Shapechange or True Polymorph into a creature that has fire immunity. Or Moon Druid 10 for elemental forms.

[tkuremento]

Ring of Fire Elemental Command
Boon of the Fire Soul
Forge Domain
Mystic's Adaptive Body

[tkuremento]

Looked through the MM for creatures that have fire immunity, though anything with a CR higher than 20 can't be used on a PC with true polymorph unless at epic levels.

Adult Brass Dragon (13), Adult Gold Dragon (17), Adult Red Dragon (17), Ancient Brass Dragon (20), Ancient Gold Dragon (24), Ancient Red Dragon (24), Azer (2), Balor (19), Barbed Devil (5), Bearded Devil (3), Bone Devil (9), Brass Dragon Wyrmling (1), Chain Devil (8), Efreeti (11), Erinyes (12), Fire Elemental (5), Fire Giant (9), Fire Snake (1), Flameskull (4), Gold Dragon Wyrmling (3), Hell Hound (3), Horned Devil (11), Ice Devil (14), Imp (1), Iron Golem (16), Lemure (0), Magma Mephit (1/2), Magmin (1/2), Nightmare (3), Pit Fiend (20), Red Dragon Wyrmling (4), Remorhaz (11), Salamander (5), Smoke Mephit (1/4), Spined Devil (2), Steam Mephit (1/4), Tarrasque (30), Young Brass Dragon (6), Young Gold Dragon (10), Young Red Dragon (10), Young Red Shadow Dragon (13), Young Remorhaz (5)