I'm a librarian, so I do get most of my books from the library. Because I order books for part of our collection, I'm always looking at journals, reviews, etc -- so I'm always filling up my tbr (to be read) list with new books. Plus, just walking around here, I see books on the different displays that catch my eye. I'm also a big believer in the "try before you buy" way of doing things.
I don't buy a lot of books for two reasons: 1) I don't have a lot of extra money to spend (Librarians don't typically make a ton of money) and 2) I don't necessarily want to own every book I read. I read a lot, and if I really, really love a book and want it for myself, I'll buy it. Otherwise, I can always check it out from the library again.
When I buy books, I try to always go through my local independent bookstore (we've got a great one with two locations near me), or I browse at the local used books store. I also really like Book Depository UK because I can get my favorite British author's books, with free shipping. I also sometimes get a free copy of a book from a publisher, author, or LibraryThing, which is always a treat.