Where to buy G series bullet train tickets from Shanghai to Beijing and vice versa?

Hello! I will be visiting in next month. i have a few questions:
1. To Beijing, I would like to go by a G series bullet train. Where can I book my tikets online?
2. With my understanding I cannot do it if do not reside in China, is it correct?
3. Are there any reliable agents available to do so for international travellers?
4. What is the probability of me getting the tickets if I am buying them 3 days ahead?
5. How long the process of booking the tickets in the station would take?

Thanks a million for answering my questions with patience.