Bus from Chengdu to Jiuzhaigou during the Golden week

Dear all,
I will certainly be around Chengdu during the Golden Week (1-7 Oct) and would like to go to JZG. Given this will be a busy time as Chinese people are travelling everywhere, how could I ensure that I can get bus tickets to go from Chengdu to JZG? Any good tips to share? Any reliable travel agent that I could contact?

Thanks for your help,
David