The best map collections for Idaho are at libraries of Boise State University and Idaho State University. The Family History Library in Salt Lake City has a small collection of maps for the territories of Oregon and Idaho.
The following publications include maps that can aid your research:
Preston, Ralph N. Maps of Early Idaho.(Corvallis, Oregon: Western Guide Publishers, 1972. Family History Library book Q 979.6 E3p). Lists early towns, gold mines, and old military roads.
An Atlas of Idaho Territory, 1863-1890. ([Boise, Idaho]: Idaho Historical Society, 1978. Family History Library book Folio 979.6 E7a).
United States Census Bureau State and County Map. This map will allow you to zoom in on any state or county in the United States and read the names of all of the neighboring counties.
Printable maps are also available from the National Atlas of the United States -- http://nationalatlas.gov/printable.html