I'm having difficult problems to get proper models of few things such as Rocks, soils, and Gravels. Photo scans are the time-consuming process. So, I'm thinking to make a community maintained project, from which people can easy download/upload his/her photo scans, for the other users which have same problems.
Let's build a Free Alternative of Megascans, (Photoscanned Library)
2 years later
Something like this has been started by the same group that created HDRI Haven. It should prove to be a very nice resource. https://texturehaven.com/
Of course it doesn't mean there can't be other sources of a similar type. If you do some research online you'll find there are many free sources of textures available. Most of them will not be PBR compliant. With the available software from companies such as Quixel you can take a non-PBR image/texture and separate the relevant channel data from it to create PBR compliant texture images.
2 years later
- Edited
Check out this site! ambientcg.com This guy has 1300 textures, some photoscanned, some made with substance, and they are all CC0 license. Definitely worth looking at if you need a texture. He used to be called CC0 textures. He also includes the substance files.
Edit: Just read your question more carefully, this guy mostly has textures for models, but he does have a few rocks and other 3d scans, either way a good resource.