As a raster analyst, there are a lot times I try to “fix” many types of raster data from various online sources. If you’re going to get data from elsewhere, there are many reliable sources available for GIS data, some websites that charge a fee, and some that are really well-done free clearing houses and data centers.
A few of my favorite national and state/regional sources for GIS raster data are:
GOS- Geodataspatial One Stop – http://gos2.geodata.gov/wps/portal/gos
USDA Geospatial Data Gateway- http://datagateway.nrcs.usda.gov/
USGS Seamless – http://seamless.usgs.gov/
GeoCommunity – http://data.geocomm.com/
EPA – http://www.epa.gov/geospatial/data.html
Great Lakes Coastal Planning – http://aqua.wisc.edu/cpr/Default.aspx?tabid=85
TNRIS – http://www.tnris.state.tx.us/DataCatalog/Index.aspx
North Carolina One Map – http://www.nconemap.com/
Minnesota – http://deli.dnr.state.mn.us/data_search.html
California – http://atlas.ca.gov/download.html
ArcGIS Online – http://www.arcgis.com/home/
National Hydrography Dataset – http://www.horizon-systems.com/nhdplus/data.php
Jeff S. - Raster Support Analyst
For Wisconsin this is a good one. http://www.wisconsinview.org/
For Montana – http://nris.mt.gov/gis/
For Arkansas – http://www.geostor.arkansas.gov
For Pennsylvania: http://www.pasda.psu.edu/
Oregon: http://gis.oregon.gov/DAS/EISPD/GEO/sdlibrary.shtml
For Alaska, check out http://www.alaskamapped.org
For Indiana: http://indianamap.org/
For Australia, see Geoscience Australia website: http://www.ga.gov.au
For New York: http://gis.ny.gov/gisdata/