E-ticket Guangzhou/Shenzhen to Hong Kong

On August 8th I'll be travelling from mainland China to HK. However in order to apply for a visa I need to proof (e-ticket) I'm leaving China. Before I'll be going to HK I'll stay in Guilin. For me it doesn't matter if I travel via GZ/SZ or any other place. I need the e-ticket within a few weeks from now, because I'll be travelling through Mongolia first and I think it's quite hard to apply for a Chinese visa in Mongolia as I don't have an invitation letter. Does anyone know where to book an e-ticket from mainland China to HK almost 3 months in advance?