In the U.S. film "Back to the Future", where does Doc Brown get the plutonium to power his time machine?

In the 1985 U.S. film "Back to the Future", Dr. Emmett Brown (Christopher Lloyd) creates a time machine that is built from a modified DeLorean automobile and powered by plutonium which Doc gets from Libyan terrorists. The Libyans or Libyan terrorists (also known as Libyan nationalists) were a group of terrorists that wanted to develop a nuclear weapon. They sought out Doc Brown for his assistance in building their device. However, Doc took their plutonium to power his DeLorean time machine. He built the terrorists a mock weapon out of an empty bomb.

The scene in the movie has;

Doc: "Oh my God. They found me. I don't know how, but they found me. Run for it, MARTY!!"

Marty: "Who? Who?!"

Doc: "Who do you think?! The LIBYANS!!!"

Doc was horrified; he realized he had been located.

The actions that Doc took caused the terrorists to hunt him down and kill him. They tracked Doc Brown to Hill Valley. In October 1985, two Libyans found him with Marty McFly (Michael J. Fox). A terrorist shot and killed Doc with an assault rifle. They also attempted to eliminate the lone witness, Marty. When a rifle jammed, Marty was able to escape the terrorists. He used the DeLorean time machine where he and the machine experienced temporal displacement. The flash of light from the machine disoriented the terrorists; it caused them to stop. Before Marty went back to the future, he wrote a warning for Doc, who wore a bulletproof vest that night he was shot and survived the attack.