Blender Insight writes:
My site has been around for a while, but I'd like to point out that I have started to create new materials that works well with Blender 2.8. I really try to get as close as possible to "real world images" and still they are free to use for you. Licensed as CC0.
First up is "wood" and "concrete", but more will come very soon. The old files for 2.7x are of course still there :).
Not only that. They are 100% Procedural and you don't download an image, but the .blend file so that you have the possibility to change the node tree to suit your specific need.