I used to buy pretty much all my books from Fictionwise.
Except some of my very first e-books, which I bought from Barnes & Noble before they closed down their e-book sales and erased the hard disks they were stored on.
Oh well. The age of books is coming to an end anyway, in any form. Soon, all text will be part of the bitstream, and only historians will care about its origin. (Although I expect religious books to be kept as talismans for some time yet.)