Where can I get a portable iPhone charger?

I'm going to a music festival and will be taking my iPhone 4(I know shouldnt take phones ect but I'm taking mine) and I will need to keep it charged, I'm going for 5 days, I need some sort of portable charger that will help keep it alive for those days, A battery powered one would be helpful(AA, AAA ECT) I will only have my phone on at night and will only be texting, any help? I want to keep it about £25 or under

Hi, this is the best for iphone 4

Maplin do a really good travel set with a normal plug, car charger and a portable charger and its not that expensive either

Probably looking at Solar Chargers - left out in the day
http://www.amazon.co.uk/...r/products

http://www.mobilesolarchargers.co.uk/

But yes there are some (normal) Battery packs that can do emergency recharge - but thats a lot of batteries to carry (and will not give the same life as a normal charge - maybe 30% charge)

maybe best policy is just turn it off (all the way off) except when using it
and power up only for the few hours you want to use it
AND keep the backlight (that eats power) set to the lowest brightness level.