Thanks for all comments
Pixel Labs has a number of free models in the C4D format. Requires sign up.
https://www.thepixellab.net/freebiesBeen experimenting with my own rock photos as image maps and using spherical projection as the image mapping (rocks are roughly spherical after all) and at the same time merging two different maps.
Regardless of projection, I find merging different amounts of two photos gives a huge range of possible outcomes because the merge can be controlled by slider or pf or distribution shader, etc.
Some of the rock test results were pretty good I thought. In this case I had what I thought was a nice set up in one file and inserted it into my Ice Cream file. The rocks changed a great deal after moving them probably due to underlying terrain being different and thus the basic rocks building off different terrain or perhaps also due to distance of the scene from xyz origin. Of course with spherical projection you place the image in the center of the sphere. With fake stones I found that centering the image a few meters above the foreground worked well enough. Detail distortion of the patterns far distant rocks doesn't show up much.
Here was my original rock test image (scene is very near 0,0,0 xyz) that I hoped to be able to move these rocks around without needing a bunch of alterations - didn't work that way, I got the lava look instead of my original even after re-adjusting the position of projected images. Need more experiments to see if this method is useful or too fiddly.
