The flexible stone walls from Chooch are the BEST! I love the look and feel of these! I wish these were around a couple of years ago when I was getting started!
Dani, I coulnd't quite make out the walls from the photo you posted, but they look a little similar to a technique I used. These are basically brick/concrete sheets from Everygreen plastics with strips of styrene thrown in every couple of inches. The benefit is that they are easy to work with, fairly inexpensive and pretty flexible:
