I'll travel from Huanglong and arrive in Songpan in the evening.

And next day in the early morning, probably 6 or 7am I'll have to take a bus to Chengdu.
I'm afraid I won't get a bus ticket. It's just impossible for me to buy the ticket at the spot in advance.
So my question is, is it possible to book the bus ticket from other stations like Jiuzhaigou? Is it necessary to book at the departure station?