This item is acquired in-game. See its sources below.
In the Appearances collection, Replica Field Marshal's Headdress is listed as Vendor. Appearance ID: 5167. 1 other item wears the same appearance:
Replica Warlord's Satin CowlRead from the World of Warcraft client build files.
Cloth is the armor type Mage, Priest and Warlock wear. With Intellect, Replica Field Marshal's Headdress suits Discipline, Holy and Shadow Priest; Arcane, Fire and Frost Mage; Affliction, Demonology and Destruction Warlock.
Replica Field Marshal's Headdress is part of the transmog set Field Marshal's Raiment (Priest only). The other 14 pieces:
Chancellor's Mooncloth Cowl
Chancellor's Mooncloth Legguards
Chancellor's Mooncloth Mitts
Chancellor's Mooncloth Shoulderpads
Chancellor's Mooncloth VestmentsChancellor's Satin GlovesChancellor's Satin HoodChancellor's Satin LeggingsChancellor's Satin MantleChancellor's Satin Robe
Replica Marshal's Satin SandalsReplica Field Marshal's Satin Mantle
Replica Marshal's Satin Gloves
Replica Field Marshal's Satin VestmentsBuilt from the World of Warcraft client build files and recorded Auction House snapshots.
Replica Field Marshal's Headdress is not traded on the Auction House, so it has no live market price. It is acquired in-game from the sources listed above. This page collects its first-party game data (identity, effects and where it comes from) from the official World of Warcraft client build.
TheWoWDB