Shanghai usually will have 20% of the tickets from Beijing to Shanghai. The rest 80% will be sold in Beijing. I know there is a trian ticket center in Jing An temple. Its on Wang Hang Du road (Between Yu Yuan Road and Beijing Road).
I am not sincere, even when I say I am not -----Jules Renard.
Dude just go to any travel agent and ask. I bought BJ-SH a while back from a travel agent, took 2 minutes, no hassle, and like 20rmb commission. Very convenient.