While there are always accusations of foreign donors funding the Iraqi insurgency it’s largest source of funds has always come from internal sources. The earliest example of that was even before the U.S. invasion had started. On March 19, 2003 Saddam ordered $1.25 billion removed from the Central Bank of Iraq. Qusay Hussein had the Special Republican Guard take away the money in tractors to the presidential palace. Saddam believed that the U.S. was only going to launch a limited military campaign like the Gulf War and that he would remain in power afterward. His greatest fear therefore was another series of internal rebellions like what happened in 1991. He therefore wanted the cash so that he could buy off tribes and local leaders to restore order and put down any revolts after the conflict was over. After the U.S. invasion American forces recovered most of the $1.25 billion, but around $100 million was unaccounted for. Some of that money undoubtedly went to fund the nascent insurgency in those early years of the U.S. occupation.
Nance, Malcolm, The Terrorists Of Iraq, Inside the Strategy and Tactics of the Iraq Insurgency 2003-2014, Boca Raton, London, New York: CRC Press, 2015
Woods, Kevin with Pease, Michael, Stout, Mark, Murray, Williamson, and Lacey, James, “A View of Operation Iraqi Freedom from Saddam’s Senior Leadership,” Iraqi Perspectives Project, 3/24/06