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