Sources for landcover:
USGS Seamless data distribution system
Natural Resources Canada at Geobase
Corine Landcover at European Environment Agency
Globcover at European Space agency
Source for SRTM THIRD:
USGS NED 1/3 at Seamless data distribution system
Sources for SRTM 1:
USA and Mexico: USGS NED 1 at Seamless data distribution system
USA and Canada: Shuttle Radar Topography Mission (SRTM) data products and non published SRTM
Canada: Natural Resources Canada CDED at Geobase
Canada: Natural Resources Canada CANVEC at Geobase
Canada: ASTER
Sources for SRTM 3:
Main source: Shuttle Radar Topography Mission (SRTM) data products
Alaska: USGS Alasaka NED 2 Seamless data distribution system
North of Canada: Natural Resources Canada CDED Geobase
Eurasia and Groendland: Viewfinder Panoramas
Antartica: Radarsat Antarctic Mapping Project