Copper plate is a crafting component introduced in the Blood and Wine expansion that can be dismantled into copper ingot. It is not needed to craft anything.
Crafting Requirements 
It only be crafted or 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