Where can I buy airport shuttle tickets when we depart from downtown?

We are going to go to the Beijing international airport by taking an airport shuttle line#5 from Huixin West Street (Under the Bridge of Huixin West Street, East of Anhui Plaza).

1) How can I buy bus tickets for the schedule? In this home page, there is the only mention about "some ticket offices in the downtown area". I want to know the exact location of ticket booth for shuttle line #5, at Huixin West Street.

2) Where is the exact spot for taking the shuttle?

Thank you for your service.

Sincerely,

Do-Hyeong