Free topographic maps are available for download from the USGS Store website. You can also find maps on other websites such as My Topo and Digital-Topo-Maps.com.
One advantage of the USGS site is that the maps are downloadable, which means you can also print them out. Other websites may or may not have that feature. Additionally, the USGS maps come from the U.S. government and accurately portray information on the land, and its maps are typically high-quality. However, some of the USGS maps may also be older maps. A smart approach to finding good topographic information is to make sure you know your source and check for dates on the maps.