Can i get a tourist visa for mainland China while visiting in Hong Kong - Macau for a few days?

Hi, I am starting my 6 week Asia holiday in Hong Kong / Macau (where I hope to spend no more than a few days) and then travelling deeper into mainland China and onwards to Shangai (about two weeks travelling time) and then continuing onwards to Korea or Japan. As an American is it possible to get my tourist visa for mainland China while in Hong Kong. And if so, how long does it take? Thank you.