Where did he get the balls to do it? He probably stole them.
Before making it as a stuntman, Knievel was a career criminal. Among other things, he was responsible for a massive, interstate burglary spree across Montana, Idaho, Washington, and Oregon. After scaling their walls, Knievel would cut a hole in the roof of his target establishment, lower himself down on a rope Mission Impossible-style, and then rob them blind.
Then after a brief, failed stint as a poacher/illegal hunting guide in Yellowstone Park, Knievel switched sides and successfully lobbied the U.S. government to allow people to hunt the Park's excess elk population, which up until then was just slaughtered and left to rot.
Only you can prevent forest fires ... caused by a guy driving a motorcycle through a flaming sign.
Not content with the most diverse resume this side of Jamie Hyneman, Knievel also started a semipro hockey team. He tried to take advantage of the 1960 Winter Olympics by inviting the Czechoslovakian team for an exhibition match, on the condition he pay their travel costs. But after realizing he didn't have enough cash to cover their expenses, he simply stole the team's receipts and refused to reimburse their money. Things got so bad that the Olympic Committee had to intervene and pay off Knievel's debts to avoid starting a war between the U.S. and Czechoslovakia's Soviet overlords. Finally, Knievel decided that if he was going to pull stupid and deadly stunts, he might as well have a bitchin' motorcycle, and the rest is history.