Fictionwise used to be one of the best bookstores on the Internet. They sell eBooks in a variety of formats. Since Barnes and Noble bought them their best sellers seem to emphasize secure eReader formatted primarily. They still sell lots of Multiformat
books too, that can be read on a variety of devices including the Kindle (you want to get Mobi format
--you tell them which format you want at checkout). The Multiformat
books are all DRM free.
There are many famous authors available in DRM-free Multiformat
such as Isaac Asimov, Ursela Lequin and short stories by the likes of George R. R. Martin--I can recommend the short story "Sandkings" if you like SF/Fantasy--but they also have many works by independent authors. They have a magazine section, most of these can be purchased through Amazon, but I haven't found some of the great short stories anywhere else.
These Multiformat Mobi
books have to be "side loaded" to the Kindle. Hop over there and have a look for yourself.