The title says it. I heard something about extracting them from a gcf file, but I'm fairly certain that's only for TF2. Here's a screenshot http://i.imgur.com/jtDXd1D.png

They're in the vpk files (newer replacement file format), and you can extract them by opening pak01_dir.vpk with GCFscape. The particle browser can't read the vpk files (Portal 2 can), but if you copy them into the particles/ folder they'll show up.