Back in the days when I had the Rocket eBook and an IPAQ, the only places you could buy from were Powells, B&N, eReader, Fictionwise and individual websites.
The joy of having an iPhone is that you can run so many different readers on it. Therefore, it really now just depends on what I want and where I can get it. If I'm buying from an independent publisher, however, then it's a totally different story. Whenever possible, I try to buy directly from the pub themselves. This allows more money to go back to the publisher and author, instead of having to pay middlemen.
Although we do sell through a LOT of retailers, and the majority of our sales happen on Smashwords and Amazon, I'm still grateful when somebody buys through our site. It really DOES make a huge difference.
Best,
Jay Hartman
Editor-In-Chief
Untreed Reads Publishing
http://www.untreedreads.com