The e-library from my local library. If I buy a book, it's normally an ebook from Amazon.
I have a few.
Thrift shops and markets are great for picking up second hand books.
The Book Depository consistently offers the cheapest new books. They're located in the UK but offer free P&P worldwide. Delivery can range from a few days to just over week.
If I want eBooks it's hard to go past Amazon-Kindle although Project Gutenberg offers a wide range of past-their-copyright books (some rare, some exceptional) from their various sites, world-wide.
And I almost forgot. If you're looking for something that's very hard to find there's a good chance you'll find it at Abebooks, which is less of a bookshop than a world-wide alliance of bookshops. At their site you have access to all their members.
I used to get tons from Doubleday Book Club. Now I buy them from Walmart and sometimes at a yard sale.
Laguna Beach Public Library's second hand store. All paper backs are 50 cents and hard backs are a dollar.
Kindle electronic book store. Buy a book and download it, all in under a minute!
Half-Price Books, Library , Used book stores, and Amazon when I want a new one right away. Just ordered three today !
My favorite place is from a friend. Not only do you get to read something new but you get to talk about the story as well.
i work at a school so the library . I go there in the morning and when I'm on my lunch break.
I don't really have time to just sit and read, so I listen to audiobooks while I do other things.