Buying HSR ticket on arrival

Hi. I am taking a bus from ZhangJiaJie to ChangSha, follow by a HSR to GuangZhou to catch a night flight in mid July.

I am checking whether will I be able to buy a HSR ticket upon arrival ChangSha for the day itself. If not, can I buy the ticket few days in advance at other station(example; Guilin)? Thanks for any info :-)