Flashpoint decorations have a chance from dropping from a certain flashpoint. They are not a guaranteed drop, but have good chances of being looted from the final boss and a high chance from the bonus boss. Hardmode flashpoints have a higher chance of dropping a decoration than storymode flashpoints.
Approximately one decoration is dropped per hardmode flashpoint, and each flashpoint drops a different type of decoration. The Banner: Czerka Sunrise decoration is dropped from the Czerka Corporate Labs Flashpoint.