SRTM data is only available in a higher resolution for USA.
Shuttle Radar Topography Mission (SRTM) data products
Source: USGS NED 1/3 at Seamless data distribution system
Source for USA and Mexico: USGS NED 1 at Seamless data distribution system
Source above 40N: Shuttle Radar Topography Mission (SRTM) data products and non published SRTM for Canada
Source for Northern Canada and void filling: Natural Resources Canada CDED at Geobase
Source for Northern Canada and void filling: Natural Resources Canada CANVEC at Geobase
Source for Northern Canada and void filling: ASTER