You'll find them all over the city of Rome. Quite often, in the form of little green kiosks that litter the historical centre of Rome, that are staffed by friendly and very helpful people.
Fiumicino: Leonardo di Vinci Airport
(International arrivals - Terminal T3) - open daily 08:00 - 20:45
Termini Railway Station - open daily 8:00 - 18:45
Fori Imperiali: via dei Fori Imperiali - open daily 9:30- 19:00
Minghetti: via Marco Minghetti - open daily 9:30 -19:00
Castel Sant'Angelo: Piazza Pia (next to the gardens) - daily 09:30 - 18:00
Ciampino Airport: international arrivals hall - daily 08:30 - 18:00