I buy from Amazon mostly, because:
- It is very convenient
- There is a lot of content, and I can usually find what I'm looking for there, and,
- Their customer service is very good,
- Their user reviews are usually numerous and helpful,
- When I buy from Amazon, I don't have to store the books on my devices. I can read them, remove them from the device, and Amazon knows I own the book and I can download it again later on the off chance I want to read it again.
I'm aware of other sources, and I downloaded a few books from them, but they are a lot less convenient for me.
It was really handy that I bought from Amazon when I started reading a book on my Kindle, and when I got an iPod Touch, I could download it again after only needing to download the free Kindle app.