I buy from
Baen,
Fictionwise and
Smashwords, and a handful of individual author sites & tiny publishers. I don't buy DRM'd ebooks. (At the moment, I don't even download free DRM'd ebooks, but that's because about six months ago I got a new computer, and I'm not yet up to dealing with DRM software. I'll probably eventually decide it's worth the hassle.)
I get free books from mobileread forums,
feedbooks, the
Baen free library, fanfiction websites like the
fanfic ebook library, individual author sites, publisher promos like the
Harlequin freebies, Smashwords, and
Project Gutenberg. I don't tend to get them from
Munseys or
Manybooks because I'm not fond of the site interface, and there's a lot of overlap w/feedbooks & gutenberg.
I'm vaguely looking for sources of free or cheap manga, but only vaguely, because I'm not strongly interested and I'm nowhere near running out of free stuff I want to read.