They are known as Bill Validators, they are hooked up in the same way as coin mechs but with a couple acceptions, they need power, which can be taken from a pc, I think, or their own adaptor, and when a bill is inserted they send pulses instead of just one button press and you can set the ammount of pulses usualy with dip switches on the side of the validator.
Validators can often be found on ebay by the way.