The Banner: Kaas City decoration can be dropped from Oppressor Alliance PacksUltimate Cartel Packs, which were released on the Cartel Market and bought with Cartel Coins. Cartel Packs, including the Oppressor Alliance PackUltimate Cartel Pack, include a random selection of items from the Oppressor Alliance Ultimate Cartel collection of items. You aren't guaranteed to get a Banner: Kaas City decoration from a Oppressor Alliance PackUltimate Cartel Pack, but it is one of the few ways to get a Banner: Kaas City deco apart from buying it on the GTN from another player.
If you're looking to get a Banner: Kaas City, 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.