What is the best kind of pastel chalk?

What kind of chalk do professional chalk artists use on the sidewalk? And where in specific can they be purch?

• Answer:

  I did a few sidewalk art competitions and found the best thing to save time and money is to brush down an underpainting of mixed powder tempera and water (like the stuff used in elementary school art class) and when that dries, layer compressed square chalk pastels (eg. Loew Cornell or Alphacolor) over it. You can get a box for five bucks or so at a local art supply or craft store like Blick or A.C. Moore. You don't have to go expensive since it will inevitably wash away!