Finding/buying train tickets in Lhasa

I'm flying to Tibet on Saturday, and want to come back to Shanghai by train on the 7th. What's the best way to go about getting tickets? Will they be terribly hard to find? Will travel agents arrange them? Do I have to queue at the station? Or are there touts who will sell them to me (and rip me off)? Is there some way I can secure tickets back now, before I get to Tibet? I asked the Shanghai-based travel agency who helped me get the air tickets and permit, and they said they couldn't help me. Thanks for any advice!

Tickets going from Lhasa to Shanghai for Sept. 7th went on sale this morning. I would recommend going to the train station as soon as you arrive in Lhasa. Chances are all the sleeper tickets will be sold out by the time you arrive in Lhasa on Saturday, but you might be able to get a seat ticket. You should be able to find someone selling tickets going to Shanghai on the 7th around the station, but they will be selling them for higher than face value.