You need to register, then click on data and it will lead you to a map of the USA with all the datasets available, from there select a region of interest, in this case Yosemite, there you'll need Point Cloud Data, it brings you to a formular to fill out, I think there you can decide to generate a DEM (digital elevation map) along with the pointcloud data, you Submit and wait a little time, once it's done processing you can download the DEM and convert it into something World Machine can read using the DEM editing software of your choice (I suggest VTBuilder or Microdem)
I've got something for you that might be a bit too specific :P You can get some very good Lidar Heightmap data from this site http://www.opentopography.org/ I downloaded the Yosemite dataset and imported the heightmap into World Machine. Your post gave me the inspiration to work on it a bit and I came up with this http://i.imgur.com/g5EbpBl.jpg with the flat mountain in the background being the Half Dome.
As for how to recreate similar terrain I recommend to strongly stylize the style, because the real terrain looks kind of boring overall, as for most real life terrains. If you want to do it all in World Machine I recommend using Voronoi cells, maybe with some deep ravines and large chunks cut off, to give them that vertical feeling. The lower portion is much easier, just try to make it interesting :)
EDIT: Just noticed that the terrain in the image linked above is mirrored, should've flipped one axis...