You can find this either from the Illinois Creek High Trail, or the trail that leads off from the Sawmill museum (trail off to the right) off Boreas Pass Road, fairly easy hike either way. Though, it is shorter to approach from the Sawmill.
You are looking for a medium Ammo can, filled with lots of trades, some bookcrossing.com books, and a trackable. I doubt you will be able to find it much into the winter, though the trail is still hikable year round.