As far as the physical books I have, most of them have been purchased at my local bookstore. The only time I've order the physical book online is if I can't find it at my local bookstore (which doesn't happen often).
As far as the e-books I have, I've ordered them on Amazon.
I mostly get my books for free from Amazon. I search for the $0.00 books and have been fortunate to find some good ones. There are free ones offered monthly to Prime members as well as the lending library. I haven't been reading a lot lately because I've been busy working on some crafts for my craft forum.