How should I do this?

So last summer my friend and I tried to make a backyard setup and it barely worked. We bought turf(not green, black and white if that makes a difference) and nailed it pretty tightly to the grass. I'm lucky because my hill is already slanted at about a 45 degree angle for 20 feet or so. On the bottom and top it goes to complete flat, so it's just about perfect for a setup. I had trouble getting speed and I came to the solution that I need to get something hard under the turf. I have no money so I don't really know the best way to do this. Where could I find wood for really cheap that would hold down?