This sounds to me like more of a terminology issue. .dem extension files are and were always fairly uncommon. However DEM (digital elevation model) data is getting more common and more widely available all the time. It comes in an very wide variety of formats (.bil, .dtd, .tiff, .adf, .bt, .flt, .dte, .ntf, .srtm and on and on) and figuring out what program can open each one is half the battle.
For TG2 use the easiest format to use that takes no conversion effort is the GeoTiff format which can be downloaded directly from the link Oshyan provided.
Other data such as the Canadian elevation data comes in a text format and needs to be converted into a file type TG2 can use.