Arabic Kindle Store
$ – Thousands of fiction and non-fiction books in Arabic are available on Amazon in Kindle eBook format. Be sure to look at sample pages before purchasing anything. Some are not formatted to display Arabic text correctly.
Dar al-Kitab al-Arabi USA
$ – Arabic bookstore in Southern California. I’ve ordered books from them before and had good customer service.
Arabic Collections Online
Arabic Collections Online (ACO) is a publicly available digital library of public domain Arabic language content. ACO currently provides digital access to over 10,000 volumes from rich Arabic collections of distinguished research libraries.