For what part of the world?
As BillB notes, the answer(s) depend upon where in the world you are working. That said, you will likely have more choices with the TERRAIN DATA category, rather than POPULAR SOURCES. There should be one or more sources for SRTM data that will cover much of the world at lower resolution (the new SRTM-Plus looks very nice from my very limited use of it). If you are working nearer to the poles, SRTM won't cover the far north/south.
For starters (at least), use World Topo Map. You can also download SRTM elevation data and contour it. Don't expect a whole lot from these sources for your area of interest. If you have other sources, they are likely to be better. I have had reasonable results from old Russian topo maps that were available without cost, for other hard-to-find areas.