Another big fantasy fan here...but I read a lot of other genre fic as well.
I purchase regularly from fictionwise.com; that was one in particular that I've seen mentioned before (I think - I mean even before this thread) and would like to put in another vote for.
And another that I buy from with some regularity, and think just from hanging around here and reading posts that other calibre users must certainly use - pretty certain I've seen a request for this one as well, though I'm not 100% sure - is
AllRomanceEbooks.com (and sometimes its companion, OmniLit; I think all of ARE is contained within OmniLit, though not vice versa, but I'm not entirely sure.)