The USGS has a website with detailed ordering info.

If you're looking to maps for boundary information, like finding private land/public land borders, the conservation department maps are probably actually going to be more accurate than USGS maps would be. Of course, for accurate terrain details, USGS maps are unbeatable.

I'll second Hazcon's advice about the terraserver site. I use it a lot for getting aerial photos of deer hunting areas, and for sort of first level "pre-scouting" for elk.