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>4.9</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.