The software link off of this page allows for downloading the tarfile.
The explicit download link is:
Once you have obtained the tarfile, you untar it in the directory where you want to keep the ATLAS source directory. The tarfile will create a subdirectory called ATLAS, which you may want to rename to make less generic. For instance, assuming I have saved the tarfile to /home/whaley/dload, and want to put the source in /home/whaley/numerics, I could create ATLAS's source directory (SRCdir) with the following commands:
cd ~/numerics bunzip2 -c ~/dload/atlas3.10.3.tar.bz2 | tar xfm - mv ATLAS ATLAS3.10.3
Before doing anything else, scope the ATLAS errata file for known
errors/problems that you should fix/be aware of before installation:
This file contains not only all bugs found, but also all kinds of platform-specific installation and tuning help.