You can't get it anymore, it was an event.
The Liberty ticket was an event item. It enabled you to go to Liberty garden and catch Victini.
>The Liberty Pass (Japanese: リバティチケット Liberty Ticket) is an event item that allows a player to travel to Liberty Garden where he or she will meet Victini in Pokémon Black and White.
Since the event passed you can't get it anymore unless you use AR codes or some other sort of cheating.
These were the times you could have gotten it without cheating:
Source: links above
Hope I helped.