Blindingly obvious answer: from an old pager. Pager motors are the motors that make pagers and mobile phones vibrate. Electric toothbrushes that just "buzz" (instead of twisting and turning the head) contain pager motors as well. Or you can make one - and motor with an off-centre weight will vibrate, and if the motor is large, you just make a large bristlebot. I have seen video of one large enough to carry an adult. It was Japanese.