Books published in 1923 or later in the United States, like the Oz books were, generally enter the public domain 95 years after publication. For example, The Cowardly Lion of Oz, published in 1923 will become public domain in 2018. (Copyright is not simple, for more information about this subject, see this informative page from Cornell.) The following Oz books are not in the public domain.
I anticipate adding these books when they enter the public domain, check back in a few years or if you can't wait, try your local library they should have these books, either on the shelf or available by interlibrary loan.