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