I get a lot of free books from here (of course),
Feedbooks and
Smashwords. Also
Project Gutenberg. Once I read a (really poorly-formatted) ebook that I couldn't find anywhere except
Google Books, but that's the last place I look. For new bestsellers, the easiest and cheapest for me is the
Sony eBook Store.
I've found that
ebookprice.info is useful sometimes.