In grade, middle and high school it was worksheets. Working a restaurant in college, it was receipts, order tickets, napkins and paper cups. Later, in an office, it was the margin of notes from countless meetings.
Doodling is a way of life, and you use what you can find. The yellow post-it note drawings of John Kenn are an excellent example of doodling at it's best.
I just want to live in this world.