If you are using Stanza on the iPhone or iPod Touch, you can just tap the "Online Catalog" button and you can browse from a wide variety of free and paid content.

For Stanza Desktop on Mac and PC, we don't yet provide any sort of in-application catalog, but you can download books from many sources to read in Stanza. See http://www.lexcycle.com/faq/where_to_get_books.