Hook: Wall Large (Purple Rectangle)The Banner: Mandalorian (Large) decoration fits into a large purple rectangle wall hook.
How to Get a Banner: Mandalorian (Large) Decoration
Cartel Pack Decorations
The Banner: Mandalorian (Large) decoration can be dropped from Revenge Alliance Packs, which were released on the Cartel Market and bought with Cartel Coins. Cartel Packs, including the Revenge Alliance Pack, include a random selection of items from the Revenge Alliance collection of items. You aren't guaranteed to get a Banner: Mandalorian (Large) decoration from a Revenge Alliance Pack, but it is one of the few ways to get a Banner: Mandalorian (Large) deco apart from buying it on the GTN from another player.
If you're looking to get a Banner: Mandalorian (Large), the easiest way to get it is to head to the Galactic Trade Network (GTN) in SWTOR and buy it with credits from another player. If it's very rare and not available on the Cartel Market, you could try buying from the GTN and hope you get it.