I want to buy a box of moon cakes. We will be docking at the Hong kong port. Where can I go close?

For the dock, do you refer to the one at Kai Tak?
If so, I think the closest place to buy the moon cake is at Store 2A of Hung Hom Market and you can take a taxi to reach there.
But to be honest, afternoon the Mid-autumn Festival, it may be hard for you to buy the moon cakes.