Hi. Good luck on your first dio.
I agree with lizardqing2. Go to Michaels, and you'll find a lot of stuff there.
Also, if you're looking for cheap solutions, look around your house, as I'm sure you'll find PLEANTY of stuff to use.
First off, you want to lay down groundwork, and what's better for representing earth than earth itself?
Mix up either very fine earth or sand with water and white glue (about 50/50 mix) and use that as groundwork. Press your models into it while still wet to represent a roadway, and leave that portion as is when dry, or you can paint it, wash it, drybrush it if you wish.
For tall grass, what a lot of people use for 1/35 scale is unravelled twine rope or the like. You can just cut it a lot shorter for 1/72. Thick bristle brushes work as well, but for your scale, again you'd have to cut it down to size.
Be creative, as almost anything can be used!!