Boarding train in Ulan Bator

My fiance and I want to take this trip for our honeymoon. We plan to leave right after Mongolian Naadam Festival and want to board at Ulan Bator (since that is where we will be). Can I purchase a ticket through a Chinese agency for a luxury sleeper or do I have to wait until the day of to find out availability? Since it is a special time for us, we would only want to travel by luxury soft sleeper. Please let me know your thoughts.