Boult and Locke

Boult & Locke are Military and Ceremonial Outfitters and are based on Peach Pie Street in Ankh-Morpork. They certainly make hats and headwear: the Postmaster General's celebrated golden winged hat was made by them.

it is entirely possible that they are also responsible for Samuel Vimes' hated ceremonial helmet, the one with the plumes.