Just saw this on Kickstarter; some extra-high capacity backup batteries that will probably be higher quality than most of the stuff on Amazon at the moment:
http://www.kickstarter.com/projects/mylimeade/limeade-blast-18000mah-ultra-high-capacity-battery
13000 mAh 11.4 oz- $59 (I see an equivalent Anker product on Amazon for $47)
15600 mAh 12.8 oz- $89
18000 mAh 13 oz – $99
