Boarback shield location

[共匪]

The Boarback shield is the best shield in the game, and you can find it after starting the band of bastards questline, behind the tent in the camp.

[Sin]

It's also available for purchase if you upgrade Pribyslavitz Smithy to Armoursmith during From the Ashes DLC.
I had both BoB & FtA DLC's underway at the time, it's possible the DLC overlap, or it's available in FtA also.