From ABINIT 5.6 and up to ABINIT 8.10.3, fallbacks (aka plugins) are automatically downloaded at configure-time. They are not shipped anymore with the sources. If you do not have a permanent access to the internet, you may of course download them now and keep them together in a directory on your computer for further use. The default directory is ~/.abinit/tarballs/, but you may store them anywhere you want. See the --with-tardir option of configure for details.
Starting with ABINIT 9, fallbacks (aka External Libraries) are no longer managed by the ABINIT build system. The abinit-fallbacks project is a package builder for the external dependencies of Abinit, in development environments lacking these components. You will find on the ABINIT wiki all the information on how to build and install fallbacks from source.
