The library is my most common source for books. It's getting frustrating, though; the branch in my town just doesn't have much I want anymore, and I don't really like driving into Fresno to use any of the branches or the main library (especially that unless you go very early or very late, it costs to park at the main library).
Then there are used bookstores and used book sales. Unlike The Master, I don't mind buying used books at all. In fact, I've gotten to prefer them, so long as there are no actual pages missing (and truthfully, I've found that more a problem buying off the bargain shelves at Barnes & Noble than I ever have in getting used books - never had a used book missing pages, but it has happened two or three times when I've bought books new but off the bargain shelves). I can't remember the last time I bought a new book at a bookstore. No, that's not true - I bought two Tim Powers books last year around this time, and at that I had to order one of them because they didn't have it in stock at the book store. The only reason I bought those was because I couldn't get them through the library and none of the used book stores I have access to had them in stock.