Brigands are located on Brigand Island. You can wait for Raiding Brigands taking over a Shire in your liberated territory and steal their Loot Wagon while defeating them. The Loot Wagon is a very popular item in battle as it may contain Shards or unavailable Upgrades for the regiment that collects it during battle.
Raiding Brigands return 2 hours after they have been defeated the last time.
Note: The Loot Wagon has to be captured before all enemies have been defeated, and you have to win the fight!
You can also choose to invade them in their home territory: Brigand Island. Win challenges to earn loot, Shards and Medals here.