In WoW, there are some quests that require you to use a tank that will appear on your character's back. Such quests can be found in Thousand Needles, and in Southshore. The goal is to obtain the quest, but not complete it, that way, when you exit the zone, the tank or "backpack" still appears on your character's back.
I obtained the blue one from Thousand Needles, but it's hard to find a set that matches.
The green one can be obtained from a quest in Southshore where you have to take ooze samples. However.. I already completed that quest awhile ago.
Does anyone know of any other places I can find a permanent green backpack?