D8 to D8 migrate source plugin?
The Drupal 8 migration module has been moved into core as explained in this issue. If you're using at least Drupal 8.6.x, you don't need to apply the patch.



Old answer:

I'm going out on a limb and say that currently there isn't one and it can only be done by creating a custom plugin. 

Just found this Drupal 8 migration (source), thanks Berdir &amp; Primsi.