If I'm not thrifting (you'd be surprised what you can find at your local bookshops or goodwills) I use amazon or ebay

If you use Bookfinder.com, they show you the cheapest books, usually around three dollars, its crazy.

I always order my books on The Book Depository. The free shipping is really useful when you live in Australia, because shipping can cost so much otherwise. Alternatively, if I just want a book but I have no set title that I'm interested, I bulk-enter Goodreads giveaways. It's nice to get a book for free.

Otherwise, the library is my favourite place in the world and I would recommend a library to anybody. You get the joy of reading the book without spending any money.