You can download a pre-built binary of plovr from Google Code. As you will see, binaries that are available for download are named with revision numbers from version control rather than with formal version numbers. This is because plovr is still in the alpha stage, as it merits more feedback from developers before the names of the configuration file properties, URLs, and query parameters can be frozen. Once developers have provided feedback and names have stabilized, an official beta will be released.
plovr is open-sourced under the Apache 2.0 license. Its source code is available on Google Code via a Mercurial repository. The plovr repository includes branches which are copies of the Closure Library, Closure Compiler, and Closure Templates source code, so building plovr is entirely self-contained:
hg clone https://plovr.googlecode.com/hg/ plovr cd plovr ant jar
The resulting plovr binary will be available at build/plovr.jar.
The process by which the branches for the Closure Tools are maintained within the plovr repository is explained on the wiki.