Your local public library is always a great source. Although the books are not free to have you can borrow as many books as you'd like for free. All you have to do is fill out for a library card (also free) and you're on your way.
Consignment stores usually carry books that have been donated. The books are not FREE, but, I have purchased great titles from the Goodwill for about 25 cents.
If you are someone who doesn't mind not having an actual book in your hands, you log on to the web and find Dickens' greatest works. Many of the sites are free and don't even require you to sign up for a newsletter. Just make sure that the site you are visiting is legit and they are making the works available with the permission of the author (or the author's estate). Always respect copyright laws.