Here are so great sites for papers. 7 Gypsies has tons of paper and other scrapbooking things (that I don't understand). My Gatsby is mainly a custom invitation store but they also sell amazing cardstock like this
I use cardstock for the mobiles that I make.
Then there is Paper Mojo. They have just about anything you could want in the world of paper.
Beautiful craft papers also make great wrapping paper for smaller gifts.