Bus from Chengdu to Songpan/Jiuzhaigou. Where I can buy the ticket online?

You can book the ticket at the official website of Sichuan Bus Ticketing System. It only has Chinese version. You may ask your Chinese friends or hotel staff to book it for you. Or, you can book the tickets on the spot at the station.

By the way, the buses to Songpan depart from Chadianzi Bus Station at 7:00, 8:30, and 11:30, and the fare is CNY 109. There is no departure in the afternoon. The last bus to Jiuzhaigou depart from Chadianzi Bus Station at 19:10, and the fare is CNY 140. :)