As for what you can use, I used this magazine series that was out in Poland that even gave you materials for drawing manga. I guess you get more of those abroad, though they tend to be pricey in the long run. I also used this book, which I highly recommend:
Draw Manga: How to Draw Manga In Your Own Unique Style:Amazon:Books It's not the best one there, but it's good as a start, IMO. It does focus on manga comics specifically in later chapters, and some of the material requirements are a bit insane (a light box? Wtf?), but you can usually work just as well without them and it really did help me establish a specific style
I can draw you some sketches of your characters if you send me their descriptions to show you how a quasi-lazy use of both sources worked for me. Considering how much I slacked off instead of practicing, I'd say the result is pretty good, and I couldn't draw at all before.