Baen:
http://www.baen.com/baenebooks
Black Coat Press:
http://www.blackcoatpress.com/e-books.html
Book View Café:
http://bookviewcafe.com/bookstore/
Booktopia:
https://www.booktopia.com.au/?ebooks
Closed Circle:
http://www.closed-circle.net/ebook-catalog/
Feedbooks:
http://www.feedbooks.com/
Google Play:
https://play.google.com/store/books
Kobo:
https://www.kobo.com/
Pan Macmillan Australia:
https://www.panmacmillan.com.au/books/
Rebellion Publishing:
http://rebellionstore.com/
Smashwords:
https://www.smashwords.com/
Soho Press:
https://sohopress.com/store/
Amazon, Apple, Barnes and Noble, Phoenix Pick are too much hassle to bother with anymore, but I'll consider buying from any other retailer so long as the end result is that the publisher gets their money and I get a DRM-free or DRM-removable ePub.