Maybe tell someone like Troy Hunt from haveibeenpawned. He has a pretty good reputation/following for verifying this kind of thing and telling the right people.
I came into here to offer the same advice, that's exactly what their specialty is.
Unfortunately, he did not answer for a week.
Started thinking data leak is not a super interesting topic for professionals.
It happened every week, so my discovery is just another data leak.
Or use one of the organizations that have a secure drop instance running [0]