For a base, you could use a small piece of plywood or any other type of wood. I usually go to the local craft store. They have many different sizes and shapes, all ready to use.
As for grass and such.... I keep a supply of Woodland Scenics material handy (the stuff used for train layouts) They have different colored grass, sand and dirt. You just apply some white glue on the wood, spread it around and shake on the grass.
For rocks and pebbles, Kitty litter is the best (clean, of course)