Ikiwiki is developed in a git repository.
The best place to download a tarball of the latest release is from http://packages.debian.org/unstable/source/ikiwiki.
Manual installation steps and requirements are listed on the install page.
Debian / Ubuntu packages
To install with apt, if using Debian or Ubuntu:
apt-get install ikiwiki
Or download the deb from http://packages.debian.org/unstable/web/ikiwiki.
There is a backport of a recent version of ikiwiki for Debian 5.0 at http://packages.debian.org/lenny-backports/ikiwiki.
Fedora versions 8 and newer have RPMs of ikiwiki available.
Ikiwiki's source includes a RPM spec file, which you can use to build your own RPM.
Ikiwiki can be installed from macports
sudo port install ikiwiki.
NetBSD and many other platforms: pkgsrc has an ikiwiki package.
FreeBSD has ikiwiki in its ports collection.
Gentoo has an ebuild in its bug database.
The openSUSE Build Service has packages for openSUSE
A PKGBUILD for Arch Linux is in the AUR.