Please, is it possible to book a bus ticket from Chengdu to Jiuzhaigou valley online ?

As I know, the booking system is in Chinese, so you may ask your Chinese friends for help. Generally, it is fine to get a tciket on the spot. Most buses are scheduled in the morning from 6:10 to 10:00. There is only one departure in the afternoon at 19:10. The bus leaves from Chadianzi Bus Station. The trip takes about 8-9 hours. :)