check at your Verizon stores for the Motorola external battery pack. The new one has a grey USB cord and sells for $40, but some stores still have the old ones with the blue cord on clearance for half the price (I ended up paying $16.50 total with my company 20% discount). They're the same exact thing as the new one but just with a blue cord. I've been happy with mine, even lent it to a few iphone guys at work to give them a charge.
Also, check stores like Office Depot, etc. in their accessories section. Yesturday I just happened to run across a "iGo" backup charger/portable battery pack (1800 mAh) for $12 on clearance regularly $40.
I tried posting links but I guess noobs on here can't post links.