There's so much stuff for CFIs on the web, you'll fry your brain trying to sift through it all. Every time I look for some specific area, I find tons more than expected. I continue to save various items for later review.
As for getting ready for your own training and your CFI practical, I'd recommend working on your lesson plan book go along with the Private and Commercial PTS books. You'll find a lot of lesson plans already written on the web but the greatest amount of my learning came from writing my own "brand" after determining what format I wanted to use. Even then, you'll find gaps. All you can do is read and read. Get quizzed and more quizzing to follow.
During the oral portion of the practical, you'll likely learn things you didn't know or thought you did. Sometimes, it's the simple things that may bite you. All you can do is learn from that, get your ticket then go learn more and more as you begin teaching.
