Best Parts look up https://www.subarugenuineparts.com/oe_parts_cat.html Best Manual http://ken-gilbert.com/wrx/impreza_manuals.htm This is opinion based naturally.
I like www.subaruparts.com, because they show a diagram. For me it is a little easier to understand, although I have used both. The manual site is excellent, and I suggest everyone go and download the manuals to your hardrive. Just try not to be a blabber mouth about the site though, because we don't want it to get shut down.