All my groundwork is actually done with earth. I mix very fine earth with a 50/50 (approximately) mix of water and white glue, then spread this mud mixture onto my dio. When dry, it looks just perfect. Anyways, you can try that. Grass, if you don't want to make your own, go to model railroad shops, as they cary a lot of modeling grass and other things for dio's.
As for pre-fabricated ruins etc, there are a number of manufacturer's. Quickest to spring to mind for me is Verlinden, but there are many others. However, be prepared to pay a hefty price.
There are even plastic parts of building ruins. I believe tamiya makes, and academy and italeri had a few as well, featuring a ruined section of wall, or a simple dorway, etc.
Check the dio forum, the people there will definitely be able to help you out.