The library is hosted on the Maven Central Repository, with the groupId org.rundeck and the artifactId apirundeckjavaclient.
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 from Sonatype Nexus OSS instance (which is then synchronized to Maven 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 "apiclientjarrundeckjavaVERSIONwithdependencies.jar")
You can download such files on GitHub : https://github.com/rundeck/rundeck-api-java-client/releases (for each release)