One already well confirmed source is Hugo Chavez. He aparently gave 6 million for the 2008 campaign and now is trying to get Obama to pay back the 6 billion dollars as a loan.
As Clinton found out with China, there's a price for foreign illegal bribes. Now Chavez wants his 60 billion dollars or he starts dropping little press releases through back channels about the money trail. If Obama doesn't cough up the 600 billion soon, to finance Chavez's secret police campaign to intimidate the Venezuelan voters he may come right out with it as revenge.