Buying tickets online for T16 train from Guangzhou to Beijing

I am living in Beijing and would like to buy two train tickets on the T16 train from Guangzhou to Beijing. But I want to buy deluxe soft sleeper for 2 tickets, so that we have a private room. Online I am not able to see this option, which web site should I visit to find these options or where in Beijing can I go to buy these tickets.