You won't find a battery to go INSIDE the unit which will last for a 3 day festival.
Some people (including me) get these little black boxes off Ebay for $10-20, you put 4 AA batteries in them, and it plugs into the iRiver, and that will power it for about a day. So you buy a big package of AA batteries and you are good to go.
Start with a freshly charged battery in the iRiver. This device won't have the power to charge a dead iriver while it's recording, but if it you start with it charged, and plug this in all day it will pretty much keep it charged.
I'm sure there are a variety of solutions out there, but this is the simplest.