Available in a thread near the top of the MC19 board. Look for "JRiver Media Center 19.0.xxx -- Available Here".
Once or twice each week, there will be a new build posted in the same location. Please download and install it.
More information is in the Mac FAQ.
Restoring a License
Here's how to Restore a License.
You can use "Update Channels" to subscribe to the version you want. Read more in Updates.
Old Versions are available.