http://www.ebooks.com 

http://books.google.com/ebooks

http://www.barnesandnoble.com/ebooks/index.asp

http://www.amazon.com/b/?&node=1286228011

http://www.fictionwise.com/ Also a B&N Store

http://ebookstore.sony.com/

http://www.booksonboard.com/index.php?ebooks-audio-books 

http://www.borders.com/online/store/Landing?view=2&type=1&kids=false&nav=5185+700152

http://kobobooks.com/

http://www.diesel-ebooks.com/publisher/E-Reads.com/results/10-Default/54.html