We can never get enough good textures!  It’s one of the things that can greatly improve the look of a model.  Ideally, look for textures that are seamlessly tiled.  That way they won’t appear to repeat their pattern or their edges. Unfortunately, one of the ways some modelers combat repeating is to just make a large texture/material cover a large area.  This can unnecessarily bloat your model size to gigantic proportions.  (Smaller-sized material images = smaller SketchUp model)

 One of the best groupings of seamlessly tiled textures are made by Patrick Hoesly.

(Go to the top and click on view all sizes, then choose an appropriate file dimension and download.

I would advise against anything larger than 640 pixels on a side.  They are seamless so you can use the smaller files!)

He is constantly updating it, so once you look through the collections, start following his blog.

Other seamless textures can be found here:

And whitewashed grunge textures here:

or has really good textures.  Just look for the ones labeled “Set Tiled” and use the smallest file size shown.