There are a number of books on Amazon.com, and I would imagine that some would probably be available through a good public library system, or in a university library where the method is taught as part of the music education program. All of the first dozen or so titles at Amazon.com are from people who are very familiar with the method, so it isn’t necessary to purchase the most expensive ones, although they are very fine books to have.
The best part of the Kodaly Method is that once you have a grasp of it you can adapt it to so many different needs in music education. I hope you enjoy learning about it.