there isn't direct bus or train from Chongqing to JIuzhaigou, but there is direct flight is $72 for a economy class, but the ticket price is changing from time to time, you may check China Flight(https://www.travelchinaguide.com/china-flights/) for more details. If you do want to take a bus to Jiuzhaigou, you have to make a transfer in Chengdu.