Super Dave,
Diodes are pretty handy devices but in relation to the robot, they can only be used in the custom circuit board or to control the function of pneumatic cylinders through the solenoids as described in an earlier post. They are available from Digikey if you have a minimum order of other stuff, or you can obtain the same part from Radio Shack. You are looking for 1N4000 series diodes. (1N4001, 1N4002, etc.)
Please note that use of the "Blue Spike" allows you to control two solenoids without using diodes.
See this site for details...
http://www.innovationfirst.com/FIRST...sersManual.pdf