Hello helpfull people!

Could someone please tell me what is the easiest way to buy train tickets from HK to Shanghai. Does Hung Hom station have special selling office for foreigners? I need to go from HK to Shanghai close to national holiday (26.09) and i'm very worried that it will be hard to get the tickets on that exact date. I'm arriving in HK on 22.09 so it's about 4 days to arrange this. I appreciate any help!