Independently developed modules are often checked into the Slicer code archives, and are tested for possible distribution in future releases.
Sometimes they are published on the Web sites of the groups that have developed them, or are announced on the Slicer users or developers mailing lists.
A new module will have to be compiled to be compatible with your version of Slicer, so you must have a version of Slicer that includes source code and .vtk header files.