The macros distributed by the Archive are maintained in a public Git repository
at <git://git.sv.gnu.org/autoconf-archive.git>. That repository can
be cloned anonymously, which is probably the most convenient way to access the
Archive. Gitweb offers an
RSS feed for subscription that informs about changes to the repository. The same
information is available via e-mail, too, on the
commits mailing list.
For the benefit of everyone who doesn't use Git, traditional release archives can be downloaded from any GNU mirror, i.e. http://ftpmirror.gnu.org/autoconf-archive/. New release are always announced on the low-traffic announcements mailing list.