You can easily make your own with project Dogwaffle, even with the free version 1.2
Here are some presets which came with a recent patch for version 2:
http://www.thebest3d.com/dogwaffle/whatsnew/2_1/Optipustics.jpg
Here's more info about the fractal particle feature of Project Dogwaffle:
http://www.thebest3d.com/dogwaffle/whatsnew/1_6/optipustics/index.html
and an example of a painting done in Dogwaffle, with all bushes , trees and grass done easily in just a few minutes using optipustics:
http://www.thebest3d.com/dogwaffle/wallpaper/wallpaperpustics.jpg
There is a fully-functional free version 1.2 here:
http://www.thebest3d.com/dogwaffle/free
However, the 'shading' feature in Optipustics is only in the commercial version. The shading adds some realism and appearnce of lightning from above by darkening the particles which are heading down.
You could use Dogwaffle to make your own grass and bushes, and save the image as a BMP file or other formats. There are examples of presets too, here in the freebies section:
http://www.thebest3d.com/dogwaffle/freebies/optipustics/index.html
Once you have saved your image (with transparency from Alpha channel, saved in Targa .tga file for instance), you should be able to use it in a 3D program. There are some tutorials showing how to in http://www.thebest3d.com/dogwaffle/tuts
I hope this helps.
-Philip