You can get radare from the website http://radare.nopcode.org/
There are binary packages for multiple operating systems and GNU/Linux distributions (ubuntu, maemo, gentoo, windows, iphone, etc..) But I hardly encourage you to get the sources and compile them yourself to better understand the dependencies and have the source code and examples more accessible.
I try to publish a new stable release every month and sometimes publish nightly tarballs.
But as always the best way to use a software is to go upstream and pull the development repository which in radare is commonly more stable than the 'stable' releases O:)
To do this you will need mercurial (a distributed python-based source code management aliased Hg) and type:
$ hg clone http://radare.nopcode.org/hg/radare
This will probably take a while, so take a coffee and continue reading this paper.
To update your local copy of the repository you will have to type these two commands in the root of the recently created 'radare' directory.
$ hg pull $ hg update
If you have local modifications of the source, you can revert them with:
$ hg revert --all
Or just feed me with a patch
$ hg diff > radare-foo.patch