There might not be anything you can do to find the right place to complain. The main issue is it is difficult to track down spam to the true sender. Spammers often commandeer the computers of innocent users to send junk mail, so simply looking at the source computer's IP address in the email message headers is not reliable (also, that can be forged). The websites promoted in spam messages often set up and shut down quickly, so if that doesn't lead you to the culprit -- and keep in mind it is safest NOT to click any such links -- you may never know.