Contact Asus for an additional or higher-capacity battery. Asus usually only makes one type of battery for any given notebook (unlike Dell or HP), but you never know. You can, at the least, purchase an additional battery.
I wouldn't get any third-party "mobile power pack". I would only buy batteries that were made by the manufacturer for your notebook - otherwise, the risk it'll fail is just too high. You can probably find a few OEM ones on Ebay, too.