Are there migration plugins for D8 sources?
Yes there is. Drupal core has a content_entity source plugin that's a part of the Migrate Drupal module. 

The one caveat with this plugin is it can only handle scenarios where the source/destination DB is the same, which sounds like your scenario here.

If source/destination DB isn't the same, the Drupal 8 migration provides a d8_entity source plugin that does support this scenario.