It should probably be taken with a grain of salt, but The Spectator (working link with the text here) suggests that CBS got The Documents from the DNC who got them from an unidentified individual.

More than six weeks ago, an opposition research staffer for the Democratic National Committee received documents purportedly written by President George W. Bush's Texas Air National Guard squadron commander, the late Col. Jerry Killian.

The oppo researcher claimed the source was "a retired military officer." According to a DNC staffer, the documents were seen by both senior staff members at the DNC, as well as the Kerry campaign.

"More than a couple people heard about the papers," says the DNC staffer. "I've heard that they ended up with the Kerry campaign, for them to decide to how to proceed, and presumably they were handed over to 60 Minutes, which used them the other night. But I know this much. When there was discussion here, there were doubts raised about their authenticity."
...
The CBS producer said that some alarms bells went off last week when the signatures and initials of Killian on the documents in hand did not match up with other documents available on the public record, but producers chose to move ahead with the story. "This was too hot not to push. If there were doubts, those people didn't show it," says the producer, who works on a rival CBS News program.

Now, the producer says, there is growing concern inside the building on 57th Street that they may have been suckered by the Kerry campaign. "There is a school of thought here that the Kerry people dumped this in our laps, figuring we'd do the heavy lifting on the story. That maybe they had doubts about these documents but hoped we'd get more information," says the producer. "If that's the case, then we're bigger fools than we already appear to be judging by all the chatter about how these documents could be forgeries."

--The Spectator (working link with the text here)

Again, take it with a grain of salt until there are more actual facts.