Powells.com sells used Spanish books online. These include novellas like Memoria de Mis Putas Tristes and Spanish lesson books, such as Breaking Out of Beginner's Spanish.
You can also buy Spanish books on Barnes and Nobles online store. eBay's online bookstore, Half.com, also has many Spanish books on sale from Spanish novels to Spanish lesson books. Lastly, Amazon.com is a great resource for finding all types of new and used Spanish books. They have a wide selection of Spanish children books, as well. For instance, Qué Cosas Dice Mi Abuela and Eris Mi Mama are popular Spanish children's books found there.