What are these websites called?

I am an American Girl collector and my collection's value will drop if I don't retrieve the boxes that the outfits and accessories in my collection came in. I want to find a website were I can ask anyone who has American Girl dolls if they can sell some of their boxes to me...but I don't know what type of website to use. Does anyone have a website I can use or know what type of website I need to use?