There are a variety of things that work. As someone already stated, poker chips are the norm. They can be purchased in a box of 100 or so. You can paint them, use sharpies on them, kind of do whatever.

At the national championship for FOM we use key chains with numbers on them.

Another FOM director uses bingo balls to draw for position and then gives out a business card of his that has his phone number in case of emergency and he writes blast off number on the back.

However, we also use weigh in slips and you must pick that up from the director or another staff member before you can weigh in. Upon late arrival the staff member will document that on the weigh in slip.