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.