Of course the green screen doesn't have to be any certain type of material, in fact there are different uses for all different shoots. For instance, I have a green fleecy for use in my indoor green screen studio whereas I have a number of muslin sheets of green screen fabric for larger warehouse and garage shoots. Other people on here have a more portable solution which is of course the flex and reversible screens. So, the color and lighting are the important aspects, look for bright kelly green or chroma-key green as these will give the best results. This is the green screen in my office
and another view
The material you use is important to be sure it's not too shiny as to reflect and cause a spill on your actors. If you are using a thin screen outside there are also going to be further complications with the screen being somewhat see through or blowing/ tearing easily. All of these aspects are important to consider. In fact, check out my new green screening tutorial videos for some more information. Part I
& Part II
You can check your local fabric store for a generally wide selection of materials and colors, so that's always a good option.