There is not a library of characters and sets available for Manga Studio 5. However, if you have access to Poser Pro Game Dev, you can export characters from Poser in FBX format and drag and drop them into a Manga Studio document. Then you can pose them in Manga Studio as needed. It's not ideal, the 3D features of the application are limited, as the program is designed for illustrators. Manga Studio currently does not offer a system for rigging your own character either. Illustudio, the Japanese version of Manga Studio had (or has, I don't know if it's still available) a stand alone app you could use to rig 3D models for use in Manga Studio but that only works with 3D models, they also have a library of content for Illustudio but to my knowledge that isn't compatible with Manga Studio. It sounds like you're confusing what Manga Studio does with what Anime Studio does. Two different programs. 

My suggestion? If you're looking to use Manga Studio but you're uncomfortable with your drawing ability is to draw. Every day. That's the only way to improve. Otherwise, you can do poses and renders in Poser, bring the images into Manga Studio and either letter those or ink over them to create your pages.