从国内去香港要办什么手续吗? Greencard holder

we are greencard holdes. Here is our plan for november trip: fly from LA to beijing and stay for a week, then go to hongkong for ~ 4 days; back to beijing stay another week, then back to LA. The ticket is very cheap, only 800$ per person, but do we need visa to enter hongknog(from beijing)? if so, can we get the visa from chinese consulate in LA? thanks.