You cannot normally buy BIRG tickets in advance, and there is no reason to do so. When you get to Civitavecchia just take the free port shuttle bus from your ship to the port entrance. Than, walk along the water (keep the water to your right) for about 4 blocks and you will see the small train station right off the sidewalk on the left side of the street (opposite side as the water). When you enter the station you will see the ticket windows towards the left side of the station and you simply tell them you want a BIRG ticket (they are 9 Euros). There are not usually long lines at the ticket counter and the workers are very good and speak some English. I am not aware of any location other than the stations that sell those BIRG tickets. You must than validate the ticket in the validation machine near the platform entrance and it is than good for round trip local trains (not the high speed reservations only IC+) to Rome and also for unlimited metro and bus use (except the Hop on Hop Off buses) that entire day. By the way, when you use the BIRG ticket in the Metro you just insert it at the turnstyles and it will let you enter.