Orichalcum plate is a crafting component introduced in the Blood and Wine expansion and can be dismantled into orichalcum ingot. It is not needed to craft anything.
Crafting Requirements 
It can be found in the reward chests for two treasure hunt quests, Applied Escapology and The Black Widow, and can be purchased from the following merchants:
- Armorer in Hauteville
- Blacksmith in Belgaard Manor, after he is rescued
- Lazare Lafargue in his workshop
- Trentin in his workshop in Hauteville
Orichalcum plates can also be obtained by dismantling magic-tier Toussaint trousers, which however can only be purchased.