*facepalm* There I was trying to figure out how it was tagged to find it and I finally got frustrated enough to google "Dean goes through mirror" lol. It's actually titled "The Mirror" http://archiveofourown.org/works/311092

Screencaps can be found here: http://www.homeofthenutty.com/supernatural/screencaps/index.php

I don't know anywhere that's specifically Cas caps. If you know the episode/scene you're looking for, it should be okay.