It's in a hut with a Rynjin sentry guarding it -- the 2nd one to the left from where the prisoners are being held as I recall but I could be wrong there. Easiest way to get to it is to come in from the Bayjin Shallows, drop into the water and run to your left at the entry. Avoid the crabs that are normally found there by waiting a few seconds until they pass by and then come up on land. Make your way behind the huts, preferrably in the underbrush, until you get past the big hut with the prisoners. It's definitely one of those behind that hut though, not in the clearing with the fire and the Rynjin party.