The USGS NED (1-arcsec) dataset is available for the continental US as well as most of Canada. It is based on the NAVD88 vertical datum.
I am interested in using the DEM to orthorectify SAR scenes in Northern Canada using the RPC model. This expects a DEM in the WGS84 vertical datum so I need to do a vertical offset of the DEM. This is possible using gdal, however, from what I can tell, the NAVD88 geoid 2012a is only defined in the continental US (plus Alaska, etc.) and parts of southern Canada.
I've downloaded the *.gtx files that are required by gdal (g2012a_conus.gtx, g2012a_hawaii.gtx, g2012a_alaska.gtx etc.), but Is there an issue with calculating the offsets in Northern Canada? Are the gtx files even defined there?