Geocode altitude of earth's surface from latitude and longitude

I'm seeking a service that returns altitude of the earth's surface above sea level from latitude and longitude. Even better would be a service that can return a polygon of coordinates at the same altitude within another polygon – although that's a big ask. Any ideas?

Google Maps doesn't appear to do this, however google maps' moon and mars maps have a colored 'Elevation' map which is "A shaded relief map colorized by elevation". Altitude could be determined by the color of the pixel in which the lat-lon is located, although this would be tedious, break easily and imprecise – and this type is not available for earth anyway..

I'm looking for ideas...