The following are resources that can help you access online YA books. You'll want to dig around: You may have to search in more than one place to find what you need!
Your local public library can also be a resource for online materials, so you may want to search that in addition to these resources. New York State residents can also apply for a New York Public Library card to access ebooks, as well.
Some YA ebooks are available in Memorial Library's ONESearch and other databases. This section includes tutorial information for how to access those materials, along with examples.
Sample ebooks from the collection:
Following are a few examples of YA books in ebook format that are available through ONESearch. Most of the available items are nonfiction, but are specifically collected to address topics of high demand and popularity.
Ancient Civilizations of Western Asia and the Mediterranean by 
Empires, Crusaders, and Invasions Through the Middle Ages by 
Exploring the Life, Myth, and Art of Ancient Rome by 
The Story of Civilization: Volume 1: The Ancient World by 