Pre-C5 the only way to get it was from the giant's cave quest.
The rewards from "Exploration of Giant's Cave, Part 1" depend on which of the four sets of items you've collected and are turning in (there's a 50/50 chance of each of the listed rewards):
Book: Giants Medical Science
60% Recipe: Tallum Tunic or 60% Recipe: Tallum Stockings
Handbook: Giants Architecture
60% Recipe: Dark Crystal Leather Armor or 60% Recipe: Tallum Leather Armor
Blueprint: Giants Golem
60% Recipe: Dark Crystal Breastplate or 60% Recipe: Tallum Plate Armor
Book: Basics of Giants Magic
60% Recipe: Dark Crystal Gaiters 60% Recipe: Dark Crystalline Leggings
In C5 the recipe you want also drops and spoils:
Drops from Hallate's Warrior, Spoils from Platinum Tribe Soldier, both in TOI.