You can buy maps online from the Forest Service here:

http://www.fs.fed.us/visit/maps

You can get USGS totpo quadrangles here:

http://store.usgs.gov/

The FS maps tend to be more up to date than USGS, but the USGS quads are much more detailed, if you are less concerned about manmade features and mostly want accurate topography.

There are many non-government topographical map publishers and most of those I've purchased maps from are reasonably good. The main thing to figure out is the scale of the map and whether that scale is suitable to your needs.