The library is hosted on the Maven Central Repository, with the groupId org.rundeck and the artifactId rundeck-api-java-client.
You can see all versions and download the files with this query : g:"org.rundeck" AND a:"rundeck-api-java-client".
Alternatively, you can download releases or snapshots from Sonatype Nexus OSS instance (which is then synchronized to Macen Central).
If you want to use this library from a scripting language, it is often easier to download a single jar file with all dependencies included (named "rundeck-api-java-client-VERSION-jar-with-dependencies.jar")
You can download such files on GitHub : https://github.com/vbehar/rundeck-api-java-client/downloads (for each release)
You can also download the latest version (not yet released) on Jenkins : https://rundeck-api-java-client.ci.cloudbees.com/job/master/lastSuccessfulBuild/artifact/target/