From what I know there is no bullet train to Leshan yet. The fastest train will get there in about 2 hrs and 10 min. You can buy the ticket at the train station or book it on http://www.12306.cn. This site requires you to register with your ID. I am not sure if you can register with a foreign passport. Maybe easier to have a Chinese friend who has an account to book it for you. Then you can get your ticket at the train station with your ID and the order number.