I have found some nice websites with a variety of free textures which range in quality, most of which can be used in Cinema 4D and most other 3D render applications.

Hopefully you will find them very useful.

 freestocktextures.com

Free high resolution stock textures, all images are free for personal and commercial use.

http://freestocktextures.com

 textureking.com

This site contains Free Stock Textures ranging from grunge, dirt, plaster, concrete, rust and more.

www.textureking.com

 texturer.com

You will find a mix of textures and photographs which are high in quality.

texturer.com

 textures.forrest.cz

This is texture library with a variety of textures such as doors, wood, water, bump maps and more.

textures.forrest.cz/

 c4d-jack.de

Nice variety of SurfaceSPREAD, SplineSPREAD, ScoobyCamTools, Seamless Reality Textures and more.

No longer exists (6 July 2018) - www.c4d-jack.de/site/en/textures.html

www.c4d-jack.de/site/en/textures.html

 paulbourke.net

This has several seamless textures varying in quality from sketched through to photographed textures.

paulbourke.net/texturelibrary/