I will show you what I prefer over the Styrofoam backgrounds that pop up in stores. I have two favorite pre-made faux walls:
1) Try searching Amazon.com for "Universal Rock" or "Universal Rocks." Cheapest place to buy is Amazon, Prime members get free shipping. All sorts of sizes, cut-to-size if too big.
2) Rocky Sheet by Designs By Nature: Rocky Sheet Link
DBN has a large range of other backgrounds, but I do like the Rocky Sheet's pricing compare to others. There are some really stunning backgrounds - they can get bulky, and they're all hand textured/colored, so they're a little pricey, but they look great and are worth the price.