If you're one of the people who keeps hearing about mythical street sweeper bristles and has never found one in the wild, consider the spring steel from windshield wiper blades.

Someone you know has wipers that should have been been replaced years ago; do something nice for them, and get enough pick-stock to last for years.

Bike spokes are also useful, but a bit more work to flatten/grind and shape. (Great for tension wrenches, though.)