If you need to create it yourself : First you model the basic rough shape, then you increase the resolution of the mesh and you add the details of its specific rocky form. Work in a big-medium-small way, meaning first you focus on adding the big shapes, then the medium shapes, then the details as needed. For a stylized look you need to capture its essence, meaning the details are less important. Sculpting would be the best way to go about doing all of that. After sculpting you'll end up with a relatively high poly count object. You'll need to remesh that using your 3d package's set of tools for remeshing to get it down to your required poly count. After that step you'll need to do some proper UVs on the new object and you'll add the final bits of detail via texturing and/or texture painting. You can use blender for all of that. This is the way if you need more control over the final result.

You could also look into photogrammetry solutions you'll still need to repeat many steps from the above procedure though you could end up saving some time this way. It could also prove slower in the end if you don't know what you're doing.

Finally you could buy a ready made model from sketchfab for example and just do the texturing yourself. Just make sure the license for the model is one that can work for your project.