booking tickets from hohhot to Ulan Bator

Hello,
I'm going to go to Ulan bator from Yanji. I found a train from Yanji to Hohhot (via Changchun) that I can book on your web site, and I found a train from Hohhot to Ulan Bator but it seems there is no way to book it, I need to have it in advance for both Chinese and Mongolian Visa. What should I do? Is there another way to do the Yanji/Ulan bator trip by train or buses ?
Thanks