Bring a 12v battery from almost anything - cordless drill, radio control car, etc...anything 12v, large enough to drive the motor, small enough to carry comfortably.
Take door panel off, provide power to motor.
Bring whatever you'll need to hook it up. I recommend two test jumpers made out of lengths of wire with alligator clips on each end. If the wire going to the motor is accessible enough you could just cut it and touch it to the battery.
Never done it before but that's approximately how I'd do it. If the guys at the junkyard are really nice and helpful they might loan you a car battery and the keys for the car you're getting it from.
Actually, you could always just use a full-service junkyard. They'll probably have a guarantee.