Written by me@grafxflow
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.
Free high resolution stock textures, all images are free for personal and commercial use.

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

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

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

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

This has several seamless textures varying in quality from sketched through to photographed textures.
paulbourke.net/texturelibrary/

Founder, Grafxflow
I am a Full-stack Developer who also started delving into the world of UX/UI Design a few years back. I blog and tweet to hopefully share a little bit of knowledge that can help others around the web. I build lighting-fast dynamic content engines with Laravel, Winter CMS and Tailwind CSS. Thanks for stopping by!