Where do atheists get their morality from?

[Angrboda]

You missed Socrates, the Eloquent Peasant, and the Code of Ur-Nammu.

And that's just off the top of my head. I'm sure research could find more. (While the Jain's reverence for the principle of no harm is certainly exaggerated, it has its roots in recognition of the effect of the Jain's action on others as analogized to harm of self.) Moreover your analysis of the golden rule's relevance to Buddhism is ridiculous; the entire metaphysics of Mahayana Buddhism is the rule writ large. The concepts of Shì shì wú ài, Anatman and Sunyata all lead inexorably to a much broader conception of reciprocity in relation to other beings than you suggest.