I've explored a map with borders such as those before. Where you can get them from, I have absolutely no clue. There was a post that contained a download of these backdrops but it was sadly removed. I do remember seeing that they were created using the basic shapes your game should come with, though, so I suppose you can always make your own. Find a mesh that has a flat surface such as your BasicCube mesh. You can retexture it with an image of the landscape you want and all you would have to do then would expand however many of those Cube meshes you need to fit the perimeter of your map. Hopefully that will serve as the "background" you are looking for.
I'll keep looking for these in case the download is still floating about. If not, perhaps someone else might know or you will just have to create your own.