You can get burlap from most fabric stores like JoAnns Fabrics. You can also order it online if you need a lot and would like to buy it in bulk. If you just cut it with scissors in what you think is a straight line, even a slight cut to the right or left of one of the threads, can cause a myriad of other threads to just fall out or unravel from the piece.
Here is the trick to cutting burlap with scissors. Choose a thread line that you want to be your edge. Pull on that thread until you have pulled it all the way out, and you have a long strip where the thread once was. You may need to use your scissors a few times to get the thread out all the way.