You should be able to find it in the root\EditorNode
get_tree().get_root().get_node("EditorNode").get_children()
The types are EditorPlugins. If you don't know which it is, then it may be worth doing print statements in the plugin's ready code, or assigning it a name, and finding it by name.
Another alternative is inside the plugin code, you can assign a reference to it using self
on any property of an object it deals with, or if there are globals that you are using.