No need to book in advance. Just get off the bus in Zhangzha Zhen, 5 minutes ahead of the destination, Jiutong Bus Station.
Legions of colorful Tibetan hostels along the riverside, Y25/bed or Y50/Rm,shared bath room and Chinese toilet. Standard hotel is Y60~Y100/Rm with attached bathroom.
Tips:
1. Don't follow any one who tries to guide you to a hotel, some of them are middleman who earn commissions.
2. Some Tibetan villagers inside the Park run hostels sneakily, it's not recommended to stay there, as it limits your choice of fun, unless you want to capture the 6:00am scene of the gully.
3. Avoid the hotels around the park, as they are usually more expensive. It costs only Y4~6 and 5 minutes for a taxi ride from the town to the gate of Jiuzhaigou Park.