The official Plan 9 sources can be found on the Plan 9 distribution site at http://plan9.bell-labs.com/plan9/. From there one can download an install floppy or a CD image.
The CD image is built every night from the latest sources and also works as a `live CD' that can be booted as a working Plan 9 system on an x86-based computer. The direct link to the CD image is:
ISO checksums: http://plan9.bell-labs.com/plan9checksums.txt
A compressed image of a bootable SD card for the Raspberry Pi is available from this link:
The CD image is also available from the following mirrors:
A shrink wrapped release of plan9 is available directly from Vita Nuova. This package containins a CD of the Plan 9 and Inferno distributions, and Plan 9 manuals and papers in the form of high quality typeset books.
Inferno is an Open Source network operating system intended for embedded applications and distributed systems created by the same people that developed Plan 9 using many of the same ideas and technologies.
It is now developed and maintained by Vita Nuova.
It can run either 'hosted' inside another os (eg., *BSD, Linux, Plan 9, Windows) or on the bare hardware.
Download page: http://www.vitanuova.com/inferno/net_download4T.html
Plan B is a derivative of Plan 9 developed by F. J. Ballesteros and his students. While the original version was based on the third edition Plan 9 kernels, the Plan B team has moved Plan B into user space so it can be run on current Plan 9 systems.