Very easy to buy the tickets using their ticket vendor. Just access through the team website. They don't operate the stadium so you can't buy tickets there. If you don't want to buy the tickets online then go to one of the team stores (shops are open on Sunday, but ticket desk is only open Monday - Saturday). Oddly enough, you should also be able to buy tickets from a tabacchi (but look for ones that sell things other than tobacco-related stuff).
Be careful about scalpers, though, as tickets have your name printed on them and since photo ID is required for stadium entry there's a decent chance you won't get in.
Team store: Piazza Colonna, 360, 00187 Roma, Italy