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