Nasa Earthdata

https://search.earthdata.nasa.gov/search?ac=true

These videos may be helpful to you

https://www.youtube.com/watch?app=desktop&v=9p_EHGxF6cc#searching

https://www.youtube.com/watch?app=desktop&v=roryIlOEWlI

Dear Mostafa,

Your question is missing more details such as How high do you want the resolution to be? for What area on earth do you want the DEM?

You can find downloadable 1-meter resolution DEM for the United States of America in the following Website

https://www.sciencebase.gov/catalog/item/543e6b86e4b0fd76af69cf4c

Hi you can download this link

https://search.asf.alaska.edu

https://glovis.usgs.gov/

(Landsat, Sentinal 2, Aster, IRS, Hyperian, SRTM etc.)

I would suggest to use SNAP, download an image that covers your area of interest (Sentinel-2, Landsat, or whatever), and use "Add the Elevation band" option. You may choose a variety of different DEMs and decide which one works better for you.