Pretty much all I buy I buy from Amazon. For selection, price and convenience.
I just recently figured out how to get epubs from certain retailers onto my kindle, but I don't see the point buying them elsewhere if they are the same price or more. I do get some books with codes from eharlequin, now that I know how to read them.
I like to have my stuff in one place and one click away on my kindles.
And I keep collecting slow shipping dollars to buy ebooks with and many sites I can earn amazon gift cards which I use for the books.
Baen looks like an interesting store, but I don't read Sci-fi and Fantasy so it doesn't do me no good.
There was a store for a while from Sourcebooks which they sold the books with no drm and they had mobi. Unfortunately they closed it.
But yeah, Amazon gets all my book budget.