There are a lot of teaching material publishers. Each one offers materials that they believe are best for you to teach from. Some are very good and some ought to be ignored. Some are very expensive and some cost little or nothing. There is nothing wrong with using someone else's materials if you can afford it and they are true to scripture.
The very best material is the Bible itself. It is suggested that you pray about what God wants you to teach and what the students need to hear at that particular time. Then study scripture and see what it says. Then see what others have to say (Study materials, commentaries, etc.) When you have finished studying, teach the Bible.
Note: There are many suggestions, hints and tips on studying Scripture in the Learning More section of this Web site.