So far the best gear I found for that slot would be from the guild armorer, which cost 5g for the level 80 rare version. Any idea if there are other sources?
Since the Lost Shores update, you can find Green, Blue, and Yellow Back Slot items as regular drops, and not just from your personal story quest.
In addition, Exotic and Ascended Backpieces are available to those who spend their time delving into the Fractals and dropping bundles of goods into the Mystic Forge.
The exotic pieces can be bought with Fractal Relics alone, but to reach ascended, you'll need a lot of stuff! This page has more info.