AmanosWorld.com: the official Yoshitaka Amano website. Very well maintained and run. It includes information on Mr. Amano, including where you can see his works in galleries; has samples of his work for viewing; has a store where you can purchase some of Mr. Amano's books, prints, and exclusively signed items; and has a fan section, including a forum, fan art section, and an e-mail address to which you can send Mr. Amano letters.
The Art of Yoshitaka Amano: a collection of selected Amano works from all sources.
A Yoshitaka Amano Shrine: contains some of Amano's Final Fantasy series artwork.
Other places to buy Amano's artbooks include Amazon.com (search for "yoshitaka amano") and Animebooks.com.