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.
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.