The main project page is at
The code is available for download at
as either a compressed tar file (tar.gz) or a zip file. (The two archives have identical contents, except that the zip file has DOS line endings.) Alternatively you can get the latest release using git
git clone -b r1.38 git://git.code.sf.net/p/geographiclib/code geographiclib
There are also binary installers available for some platforms. See Using a binary installer.