It's called the Office of Special Counsel [osc.gov]. The Office of Special Counsel provides "a safe conduit for the receipt and evaluation of whistleblower disclosures from federal employees, former employees, and applicants for federal employment."
The law protects whistleblowers, the question is whether Manning is a whistleblower. A whistleblower is someone who tells the public or the authorities about corrupt or illegal behavior. Little if any of what Manning exposed qualifies as corrupt or criminal, so he's not protected as a whistleblower. Even the most famous release, the "collateral murder" video of an Apache attack helicopter slaughtering journalists in Iraq, wouldn't qualify because it was an accidental killing; it doesn't even qualify as negligence, since the pilots and the military can argue that when journalists are embedded with heavily armed insurgents carrying AK-47s and RPGs, they can hardly be expected to recognize them as press. I do think Manning did a real service in releasing this video- it shows the real costs of war in the most horrifying possible way, something we should remember before we decide to plan another invasion. But unless humanity gets together and decides to outlaw war and make civilian casualties illegal, exposing the brutality and tragedy of warfare does not qualify as whistleblowing.
Even if that incident or other incidents did qualify as whistleblowing, it wouldn't get him off the hook however. Snarky comments made by U.S. diplomats don't qualify as corrupt or illegal, so there is no chance that his lawyer can argue that releasing those cables was justified under a whistleblower law. I sympathize with his aims and his treatment may be excessive, but it doesn't change the fact that he broke the law.