For Verizon HTC Droid Incredible 3500mAh Extended Battery Cover US Seller | eBay
I have built a recharge station for when I am on long trips and want to watch netflix, its basically a small 12V sealed lead acid with a 12V charger. A simple solar charger can recharge the 12V battery on the go too.
There are tiny 12V batteries out there, like this one if you want small: National Battery 12V 1 2AH Premium AGM SEALED Lead Acid Battery | eBay
And there are bigger ones depending on what you want/need.