The SPARK 2014 project has been open source from the beginning. It's source code has been accessible until very recently on the open-do.org forge.
However, since today, SPARK2014 and all its components why3, alt-ergo, cvc4 and z3, are available on github. This allows for a stronger interaction with the community, which is already on github for the most part. In this way SPARK2014 joins numerous other AdaCore projects, which are already on github, such as gprbuild, gtkada, gnatcoll and xmlada.
You can clone the whole project including submodules using
git clone --recursive firstname.lastname@example.org:AdaCore/spark2014.git
Feel free to clone, open issues, contribute!