Sketchpad may be purchased at http://dynamicgeometry.com/
However, if you do purchase it, please be aware that you are purchasing Sketchpad 5 and our computer lab is using Sketchpad 4. As a result, if you create a file at home, you must save it as a "GSP 4 document (compatibility version)" or you will not be able to open it in the computer lab. There is no problem in the reverse direction--Sketchpad 4 files created in the lab can be opened in Sketchpad 5 at home.
If you have any questions about this, see the instructor.