Xian from Beijing plus Terracotta Warriors and Horses

We are planning to see the Terracotta Warriors and Horses, we would prefer not to do with a tour if at all possible. We would like to take the bullet train which in my understanding is about 5 hours from Beijing which is where we are staying. However, where can the tickets for the Terracotta Warriors be purchased online?, so that we have them in advance being that there is a limited amount of entries allowed each day since the new ticket purchase policy was implemented back in October 2017.