Hi and thanks for the website. Is there any option to purchase GZ>SZ tickets online?

We'll be staying at Private Enjoy Home Apartment (The New Pearl River Shore Apartment), and want to take the 9:00 am express train to Shenzhen (less than 40mn it seems).
But I can't find any option online to book the tickets in advance, what are our best chances to buy the tickets?
Thanks again!