The U.S. Geological Survey has put its entire current topographic map collection on the internet as PDF files. The maps are available at scales of 1:24,000 (1 inch = 2000 feet), 1:100,000 (1 cm = 1 km), and 1:250,000 (approx. 1 inch = 4 miles). There are other sites (such as TopoZone) that offer a seamless database of USGS topo maps, but the USGS site has the entire map, including the border information, and the scanning resolution is almost as good as having the actual piece of paper in your hands.
Montana 1:24,000 topographic map, Granite Peak:
Detail from the Montana Black Pyramid Mountain topographic map:
To download these maps, go to the USGS store, and click on the Map Locator & Downloader picture:
Grace and Peace