Where will you be coming from and how will you be getting to the Little Naches area?
Also when do you plan on going?
Most of the area is all National Forest and no pass is required. But, if you are heading over from the West side of the state by taking the historic Naches Pass Trail, you will need the NW forest pass on the west side of that trail.
It has been years since I lived over on the West side of the state, but if they are anything like the Discover Pass, you can pick them up at any place that sells license tabs, or fishing licenses. Walmart even sell Discover passes.
Just in case you did not know already, all the trails in the Little Naches area are closed until June 15th these days. You can travel on Forest Service roads, but getting caught on a trail before they are open can get you a pretty hefty fine.
Little Naches is my favorite place. So many places to camp. Enjoy, have fun, happy Jeeping!