In Harry Potter 7, Snape mentions a secret source to Voldemort, WHO is it?

I agree with you that it is not Dumbledore (though it was). As for Dung, I think we have to acknowledge the fact that Lord Voldemort does not really think highly of the other Order members. They would be viewed by him as not so different from Dung. I believe Lord Voldemort did trust Snape's urances that Dung's information were reliable. Judging from how Snape was able to hoodwink the Dark Lord, it's not impossible that he could get away with this.