the silver dragon scale is likely to come off of a dragon... possibly after a fierce battle. he's probably tough to get to. ask around to the other dragons you find for some guidance.
that mirror... hmmm.... not sure... but i believe theres a couple of brothers near evenshire that keep up with some odds and ends as such. id ask them, regardless of how stubborn they can be.
for sensei philter potion, one must seek a sensei at dusk.
Courage doesn't always roar. Sometimes courage is the quiet voice at the end of the day saying, "I will try again tomorrow. Especially if it's trying for the $#%% bulletproof gaunts"