The National Map is the primary source of the National Elevation Dataset (NED) data of the U.S. at: http://nationalmap.gov/viewer.html.. When you first visit this site you should read the directions for downloading data from the National Map Viewer before searching for data. This will teach you to easily navigate the site and select exactly what you need. These data have a Geographic “projection” (latitude/longitude) and use the NAD83 datum. You will find that there are many other types of data available at this site. You should explore this site at your convenience.
Within the U.S. you can access NED data at 1 arc second (~30 m), 1/3-arc-second (~10 m), and in some locations 1/9-arc-second (~3m) resolution. For Alaska, the NED data are primarily 2-arc-second (~60 meters). All of the data are prepackaged in 1x1 degree tiles, except for 1/9 arc second data that are prepackaged in 15x15 minute tiles.
Zoom in to your area of interest and click on the Elevation Availability in the Table of Contents on the left. If a certain checkbox is disabled or an error notification box appears, then zoom in for finer spatial resolution. Reminder: the data is available for only North America (U.S., some Canada, and Mexico).
Once you have confirmed the dataset you are interested in is available, click on the Download Data button on the toolbar or in the upper right of the window. There are a few options for defining your region of interest; draw a rectangle; select a reference area, adjust current map extent, or enter coordinate input. Select a method then specify the type of data you would like to download, in this case Elevation, and click Next.
You are presented with a list of tiles at different resolutions, in different formats. Currently the output formats available on the National Map Viewer are IMG, ArcGrid, and GridFloat. You should choose the IMG format if working with ENVI.
Check off the product(s) you are most interested in and click Next. The Table of Contents pane changes to a view of your Cart. Check out by providing your email address. The link (or multiple links depending on the size of the region) to download the data will be sent immediately to your email address.