The US Geological Service is the "official" distributor of maps from most of the agencies involved in any sort of charting (including air shots), although some agencies also distribute via their own offices. Most of the shots available are a few years old, so very recent stuff may not be released there.
Google also provides pretty good ones, but they won't allow me to connect via dialup. (They block downloads by slow people.) With Google you can browse to the place you want, and select the "zoom level" to get particular levels of detail - at least I've been told.
It you're looking for something for a wall hanging, you'd probably be best suited with something from a printer, rather than a download. If you're looking for information about your neighbor's nighty then Google is probably your best bet since it lets you browse and zoom.