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