You can go by bus or airplane. If the weather is not good, for it may snow. Then, suggest you take airplane. The bus will takes 10 hours and the road cover with snow makes the bus slower. As I know, if you buy the air ticket now, it has the ticket in Mid-December costing no more than 1000RMB. You can keep checking China Flights(https://www.travelchinaguide.com/china-flights/) information online to pay attention to the ticket price. Book when you think the price is reasonable.

Thanks Mr Manny...How about budget accomodation in Jiuzhaigou? Is there any?

Jiuzhaigou has intel. youth hostel which is just near to the entranve of Jiuzhai Valley. Or 友贵贵宾楼which is 600 meters from the entrance. Star hotels are usually far from the valley entrance.

The Sheraton is quite close to the park entrance and is a lot cheaper at that time of year. Flights are also only around 600 RMB each way. Check for accommodation as well as other hotel websites.