YouTube - Mouse CO2
YouTube - mini Co2 mice, rat euthanize Kit

Honestly, IMO, Co2 gassing can't be humane. They're aware of what happens and suffocate. Maybe it's not as bad as having your throat smashed in, but suffocation is suffocation. For me, I would go the way of cervical dislocation or a smack to the head unless I was dealing with large rats or LOTS of rodents.