Each vehicle parked at a trailhead requires a Northwest Forest Pass. Passes cost $5/day or $30/yr. and can be purchased at REI, the Glacier Public Forest Service Center(open daily mid-June through September, 8 a.m. - 4:30 p.m.) from a vending machine at the Mt. Baker ski area, at the Baker Visitor's Center (open late July through Labor Day) and many other locations.
The Mt. Baker-Snoqualmie National Forest website provides additional information on the passes and locations where they can be purchased.