Murray Waas, who has an excellent track record for these things, reports that Novak’s source regarding Valerie Plame was Karl Rove.  Moreover, he reports that three days after the announcement of a federal investigation into the leak, Rove and Novak spoke by phone and — some investigators believe — concocted a cover story.  (What this means in legal terms?  The two engaged in a felony known as "obstruction of justice", among other things:

On September 29, 2003, three days after it became known that the CIA had asked the Justice Department to investigate who leaked the name of covert CIA officer Valerie Plame, columnist Robert Novak telephoned White House senior adviser Karl Rove to assure Rove that he would protect him from being harmed by the investigation, according to people with firsthand knowledge of the federal grand jury testimony of both men. . . .

Rove and Novak, investigators suspect, might have devised a cover story to protect Rove because the grand jury testimony of both men appears to support Rove’s contentions about how he learned about Plame.

Below is a picture of the two together.  Rove’s pin says, "I’m a source not a target" (looks like he’s both):