The Guardian Set currently consists of Guardian Shield, Guardian Halberd, Guardian Axe and Guardian Boots.

Guardian Shield comes from Demon Skeleton, Diabolic Imp, Energy Elemental, Fire Devil, Eternal Guardian and Mutated Tiger.
Guardian Halberd is available through a quest and also dropped by Annihilon.
Guardian Axe is lootable from Deepling Guards and Tyrants. It is also a reward from Jaul's, Tanjis' and Obujos' chests.
Guardian Boots are lootable from Ghastly Dragons and their bosses.