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