็How to buy the Bus ticket from the agency

I will arrived Chengdu East Railway Station at 22/10/2015 and to go to Jiuzhaigou at 23/10/2015.
According your information about the ticket agency at the east railway station (East Railway Station: Zone 01, Platform Mezzanine). How to get them when I arrive? They are stay in the station or i will go to outside the train station. Because I want they to reserve the bus ticket for me. Thank you