The Mandalorian Weapon Rack decoration can be dropped from Outer Rim Explorer's Packs, which were released on the Cartel Market and bought with Cartel Coins. Cartel Packs, including the Outer Rim Explorer's Pack, include a random selection of items from the Outer Rim Explorer's collection of items. You aren't guaranteed to get a Mandalorian Weapon Rack decoration from a Outer Rim Explorer's Pack, but it is one of the few ways to get a Mandalorian Weapon Rack deco apart from buying it on the GTN from another player.
If you're looking to get a Mandalorian Weapon Rack, 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.