I buy from Fictionwise, mostly.
However, I have bought and otherwise downloaded occasional books directly from the author. I'm currently enjoying Jonathan Feinstein's Downtime series...
http://www.sc2.com/e-books/
Possibly not what you're looking for in a business model, since he is giving away the ebooks in order to promote the dead tree versions; however, his nominated charities are going to do very well out of me.