Supposedly, the memos came from Killian's "personal files". However, today his son denied this. And logic precludes this, as these memos are evidence of the alleged author's unethical behavior. Who saves for posterity evidence of one's unethical behavior?
So, in answer to your question, I believe the "memos" were provided by Paul Begala and James Carville who, knowing the documents would sink the Kerry campaign, are paiving the way for a Hillary Clinton presidential run in 2008.
Yep, Hillary pulled down that man's pants. Bet that felt strange to her.