Releases of JOpt Simple are synced to the central Maven repository. Declare a dependency element in your POM like so:
...
<dependencies>
...
<dependency>
<groupId>net.sf.jopt-simple</groupId>
<artifactId>jopt-simple</artifactId>
<version>6.0-alpha-3</version>
</dependency>
...
</dependencies>
...If you're not using Maven, you can still download the newer releases from the Maven repository manually, here.
You can find older releases at SourceForge.