I'm trying to shorten my 'to-read' list, so I haven't bought many books lately. But when I buy, I usually look for the cheapest. I de-DRM my books as I want to be 100% certain I can access them in the future, so where I buy doesn't matter as much. Most often I buy from either Amazon or Cyberread - because at Cyberread you can get 40% off an order for writing a review (
http://www.cyberread.com/book-reviews), and that make their prices quite competetive.