We did not purchase the Roma pass until we arrived either. The best place to buy one happened to be at the Tabaccchi store in the Metro station across the road from the Colloseo. IIRC it's cash only, however. A word about the buses, everytime we rode/saw them they were packed! Fine for getting around but may not be the easiest to see sights from when they're jammed together like sardines in a can.