Where is the folder with all the textures such as black wall tiles, white portal tiles, etc. in the portal 2 directory? I need one for a model i'm making. Thanks!

If you select one of the textures in Hammer's texture browser it should display the VPK-relative path.

You also need GCFScape to access files inside the VPK, if you want to preview the texture while you're modelling.

If you open pak01_dir.vpk using GCFScape, you'll find a directory structure similar to the portal2 one. You can then extract any file you need from there.

Beware though, that if you want to use a brush/world material on a model, you need make a copy of it and change the material type. (Read the materials section on the VDC for more info.)