Recommended: The Paper Mill Store & Green Field Paper Company
They also have a bunch of free templates available for download here.
If you want to go completely treeless (& have a little more $ to spend), take a look at Green Field Paper Company, for writing, cover, and drawing papers made with hemp fiber and recycled papers with up to 75% pcw content. They've also got a line of decorative handmade papers, made with garlic, coffee chaff, and even 100% recycled junk mail.
UPDATE: Scott Pierce points to Vision Paper for tree-free options as well. (Thanks, Scott.) I've written for sample swatches and a price list. Their papers are made from kenaf, a member of the hibiscus family.
ANOTHER UPDATE: I received my order from the Paper Mill Store . . . along with a sample mini pack of some beautiful glossy Kromekote by Smart Papers as a free sample. They rock.