It's a separate download. If you're using a build tool which manages dependencies (which i highly recommend!), you can get it from Maven Central at javax.comm:comm:3.0-u1.
As well as the API library itself, you will need an implementation. The de facto standard seems to be RXTX, which is also available via Maven Central. For reasons best known to themselves, the RXTX developers have actually stopped supporting the javax.comm
API; the latest release that supported it was 2.0.x. That is available on Maven Central at org.rxtx:rxtxcomm:2.0-7pre1.
If you don't mind giving the up the standard API, there are also newer releases at org.rxtx:rxtx:2.1.7. Apparently, the new API is the same, just under a different package name. Which makes it even more baffling that they changed it!