Train ticket to Tibet

Hi,

We want to book our ticket from chengdu to lhasa with you.
Is it guaranteed we get the ticket we want ( soft sleeper...we understood they are hard to get..).
We want to travel on the 29th of july.
Can we buy the tickets now online? Or do we have to wait untill they are availible? And when are they availible...can we reserve them?
We have booked an organized tour to Tibet from Chengdu and we have reservation in a hostel there. Can you deliver the tickets there?