From the Hill:

The source who in July gave news media Rep. Mark Foley’s (R-Fla.) suspect e-mails to a former House page says the documents came to him from a House GOP aide. That aide has been a registered Republican since becoming eligible to vote, said the source, who showed The Hill public records supporting his claim. The same source, who acted as an intermediary between the aide-turned-whistleblower and several news outlets, says the person who shared the documents is no longer employed in the House.

UPDATE: This accords with what Brian Ross has been saying, for what it’s worth. From the New York Times the other day:

Mr. Ross dismissed suggestions by some Republicans that the news was disseminated as part of a smear campaign against Mr. Foley.
“I hate to give up sources, but to the extent that I know the political parties of any of the people who helped us, it would be the same party,” Mr. Ross said, referring to Republicans.