Below you can find a tutorial-like content on Importing DEM Files for Terrain.
This website has most of the terrain data worldwide: https://earthexplorer.usgs.gov/
1- Note that sign-up needed for downloading data.
2- Search for your location.
3- Choose “ASTER GLOBAL DEM” or “SRTM 1 Arc-Second Global” as your data. (Not sure which one is better but i think SRTM is newer)
4- If there are more than 1 option to choose, download GeoTIFF format (then convert it to USGS ASCII DEM file format that SketchUp reads natively with software called 3DEM. This software reads/converts/saves different types of height data and let’s you crop those data maps (When data is so huge, SketchUp crashes).
5- Import DEM file to SketchUp (don’t forget to click options and increase points number for higher detail)
You can clean the mesh with CleanUp³ extension.