Can't recommend the tutorials here enough, even the ones without the high ratings will likely have something to offer.

The majority of my textures come from either Spiral Graphics (their Genetica viewer is free and allows limited manipulation of the textures included) or CGtextures.

I've also gotten oodles of good stuff from the Dundjinni User Submission forums. There are plenty of other sites as well, just to do a GOOGLE search.