Can I purchase the Shenzhen visa a day earlier at HK if my tour agency seem to insist that I have to

Tour Agency in Singapore charging $90 to get it to Shenzen. I am an Australian and have the misfortune of getting my friend to pre book my Hk/Shenzhen trip and may be stuck without a visa. As I will arrive a day earlier in HK and my tour agency City 99 Asia is not helpful can you pls advice. Also what documentation is required. Thanks