I´m not sure there is any special festival power. (Apart from beer perhaps
)
Anyway, if you get two 6000mAh batteries together they would give you more than 10 hours continous recording, should last a festival.
Do not buy the Sony ones branded InfoLithium unless you want to pay serious brand premium (and for the InfoLithium feature that the 722 does not us).
You could bring a car charger cable if you wish, nothing fancy as the 722 can accept the nominal 12V directly. It might be a good idea to buy the cable from SD as it is a bit of a pain to solder in the very tiny power contact.
Gunnar