Mechagon Armory Rack

To collect this, you need to have (on at least one character):

1. 
   
2. Opened up BfA (if you've done it on one character, you can skip the entry quest by selecting the option to skip the scenario/play-through instead of completing the third quest - fly to Stormwind for Horde, teleport to Boralus sewers for Alliance)
3. 
   
4. Opened up Mechagon by completing the The Legend of Mechagon / The Legend of Mechagon quest chain starting near the Flight Master with the gnome/goblin near the flight-point in Boralus/Dar'zalor
5. 
   
6. Completed the Mechagon entry quests up to, and including, activating [npc=150359]
7. 
   

You might want to look at the Mechagon Junkyard-Tinkering Guide for further details. Especially if you part-completed the quest chains, but dropped them somewhere along the line.

Décor items also are random drops from the daily quest boxes and Rares.