In the name of our great master... For the glory of Illidan!
The common components for the soul cannon are obtained through various trades. Engineers and miners should be able to provide you with the felsteel bars, khorium power core and adamantite frame.
For the flawless arcane essence you must travel to Terokkar Forest. In the center of the forest, above the Horde outpost of Stonebreaker Hold, you will find the home of Sar'this, an arakkoa heretic. The area is known as Lake Jorune.
Speak with Sar'this and tell him what it is that you seek. Be certain to mention that I sent you as he owes me a favor or two.
Do as Sar'this asks and you should have the essence.