Hook: Wall Large (Purple Rectangle)The Banner: Sith Empire decoration fits into a large purple rectangle wall hook.
How to Get a Banner: Sith Empire Decoration
Cartel Pack Decorations
The Banner: Sith Empire decoration can be dropped from Wild Space Explorer's Packs, which were released on the Cartel Market and bought with Cartel Coins. Cartel Packs, including the Wild Space Explorer's Pack, include a random selection of items from the Wild Space Explorer's collection of items. You aren't guaranteed to get a Banner: Sith Empire decoration from a Wild Space Explorer's Pack, but it is one of the few ways to get a Banner: Sith Empire deco apart from buying it on the GTN from another player.
If you're looking to get a Banner: Sith Empire, 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.