Forest Service "Visitor" Maps are $9-10 and there is one for every National Forest in the U.S. Sometimes National Forests have 2-3 maps - for example, the Los Padres National Forest has two maps because it covers such a huge area. Maps are the definite source for Forest Roads and Facilities. Also included are trails, but these maps generally aren't topographic.
The Forest Service also produces maps for many of its wilderness areas. These generally provide a better scale for wilderness trails and are topographic.
To purchase maps, visit your nearest ranger station or go online to www.nationalforeststore.com You can also buy annual passes here.
Region 5 (California) also has its forest maps available online at this link.
Also check out this link for other forest maps that may be available online.