Detail this asset from Unity Store: Original Link
This is a paid asset, but now you can download Nature Environment Free.
Nature Environment v1.0
Nature Environment asset created for quick and quality development of exterior scenes, natural landscapes and outdoor locations.
23-Full editable tree,bushes and plants
27-Textures of grass
20-Seamless texture ground with normal map
7-Model rock and boulders with PBS
4-Model stump and dead tree
If you need small stones, you can free download them here