Neo4j Spatial (later NS) is a plugin, like APOC, discussed in Chapter 5, Awesome Procedures on Cypher - APOC. The homepage of this plugin is https://github.com/neo4j-contrib/spatial.
Just like APOC, you should take care to install a version compatible with the version of your Neo4j server. See the releases tab on their GitHub landing page. The release notes and the release filenames are self-explanatory. (Thank the project leader, Craig Taverner.)